Inconsistency, NaT included in result of groupby method first but not NaN
`NaT` is included in result of `groupby` method `first` while `NaN`. I am expecting that  first should skip both `NaN` and `NaT` and include the first value where `pandas.isnull` is False.
Demonstration of the inconsistency. (note that both `NaT` and `NaN` in the data frame are produced by `np.nan`, the difference is that the d_t column contains date values).

``` Python
import numpy as np
import pandas as pd
from datetime import datetime as dt

testFrame=DataFrame({'IX':['A','A'],'num':[np.nan,100],'d_t':[np.nan,dt.now()]})
```

Resulting data frame:

```
  IX                     d_t  num
0  A                     NaT  NaN
1  A 2015-07-15 22:47:10.635  100
```

Grouping this data frame on the `IX` column and executing the `first` method results in this data frame which shows the inconsistency between the `d_t` and `num` columns.

```
testFrame.groupby('IX').first()
```

Resulting dataframe:

```
        d_t  num
IX              
A       NaT  100
```

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.9.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.16.2
nose: 1.3.4
Cython: 0.22
numpy: 1.9.2
scipy: 0.15.1
statsmodels: 0.6.1
IPython: 3.0.0
sphinx: 1.2.3
patsy: 0.3.0
dateutil: 2.4.2
pytz: 2015.4
bottleneck: None
tables: 3.1.1
numexpr: 2.3.1
matplotlib: 1.4.3
openpyxl: 1.8.5
xlrd: 0.9.3
xlwt: 0.7.5
xlsxwriter: 0.6.7
lxml: 3.4.2
bs4: 4.3.2
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 0.9.9
pymysql: None
psycopg2: None
```
