BUG: bool(NaT) was True
bool(NaT) evaluates to True. I am not sure if this is by design or if it is a bug, but I think it makes more sense to evaluate it to False.

`bool(nan)` is also `True`, and I think it should be consistent with that. Here's the python docs on what's considered False: http://docs.python.org/2/library/stdtypes.html#truth-value-testing
 This type of testing is for compat with numpy and python as @TomAugspurger points out.
use `pd.notnull/pd.isnull` for reliable/consistent testing on all structures / values

```
In [1]: bool(np.nan)
Out[1]: True

In [2]: bool(np.datetime64('NaT'))
Out[2]: True

In [3]: bool(pd.NaT)
Out[3]: True
```

```
In [8]: pd.notnull(np.nan)
Out[8]: False

In [9]: pd.notnull(pd.NaT)
Out[9]: False

In [10]: pd.notnull(np.datetime64('NaT'))
Out[10]: False
```
 Thanks for setting me right :)
I'll close the pull request.
