sort_values not implemented for Panel
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
pd.Panel(np.random.randn(2, 5, 4), items=['Item1', 'Item2']).sort_values(by=0)

---------------------------------------------------------------------------
AbstractMethodError                       Traceback (most recent call last)
<ipython-input-16-f14e23e389bd> in <module>()
      1 import pandas as pd
----> 2 pd.Panel(np.random.randn(2, 5, 4), items=['Item1', 'Item2']).sort_values(by=0)

/home/gpepe/software/anaconda3/envs/science/lib/python3.6/site-packages/pandas/core/generic.py in sort_values(self, by, axis, ascending, inplace, kind, na_position)
   2010     def sort_values(self, by, axis=0, ascending=True, inplace=False,
   2011                     kind='quicksort', na_position='last'):
-> 2012         raise AbstractMethodError(self)
   2013 
   2014     _shared_docs['sort_index'] = """

AbstractMethodError: This method must be defined in the concrete class of Panel

```
#### Problem description

It seems like the sort_values() method is not implemented in Panels, but it is not clear from the raised error message. There seems to be a page on this in the API reference: http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Panel.sort_values.html. i would find it highly useful to have such a method.

#### Expected Output

Sorted panel

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

<details>

INSTALLED VERSIONS
------------------

commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 3.16.0-4-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.utf8
LOCALE: en_GB.UTF-8

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None

</details>
