PanelGroupBy.count not implemented anymore
It seems that the `count` method no longer works on `PanelGroupBy` objects, instead raising `NotImplementedError`?

```

dr = pd.date_range("20160505", "20160507 23:00", freq="H")
test_panel = pd.Panel(data=np.random.random((2,72,2)), major_axis=dr)
gb = test_panel.groupby(pd.TimeGrouper('D', axis=1))

# works under all versions
gb.agg(pd.Panel.count) 

# raises NotImplementedError under 0.17.1 and 0.18.1
# pandas 0.16.2 gives same result as previous line
gb.count() 

```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS                                                                                   
------------------                                                                                   
commit: None                                                                                         
python: 2.7.11.final.0                                                                               
python-bits: 64                                                                                      
OS: Windows                                                                                          
OS-release: 7                                                                                        
machine: AMD64                                                                                       
processor: Intel64 Family 6 Model 62 Stepping 4, GenuineIntel                                        
byteorder: little                                                                                    
LC_ALL: None                                                                                         
LANG: None                                                                                           

pandas: 0.18.1                                                                                       
nose: 1.3.7                                                                                          
pip: 8.1.1                                                                                           
setuptools: 20.7.0                                                                                   
Cython: 0.24                                                                                         
numpy: 1.10.4                                                                                        
scipy: 0.17.0                                                                                        
statsmodels: 0.6.1                                                                                   
xarray: 0.7.2                                                                                        
IPython: 4.2.0                                                                                       
sphinx: 1.4.1                                                                                        
patsy: 0.4.1                                                                                         
dateutil: 2.5.2                                                                                      
pytz: 2016.3                                                                                         
blosc: None                                                                                          
bottleneck: 1.0.0                                                                                    
tables: 3.2.2                                                                                        
numexpr: 2.5.2                                                                                       
matplotlib: 1.5.1                                                                                    
openpyxl: 2.3.2                                                                                      
xlrd: 0.9.4                                                                                          
xlwt: 1.0.0                                                                                          
xlsxwriter: 0.8.5                                                                                    
lxml: 3.6.0                                                                                          
bs4: 4.4.1                                                                                           
html5lib: None                                                                                       
httplib2: 0.9.2                                                                                      
apiclient: 1.4.2                                                                                     
sqlalchemy: 1.0.12                                                                                   
pymysql: None                                                                                        
psycopg2: None                                                                                       
jinja2: 2.8                                                                                          
boto: 2.39.0                                                                                         
pandas_datareader: 0.2.1 
```
