API: unpickle problems with pd.NaT
Error when reading a pickled pd.NaT value.

Example

``` python

In [1]: import pandas as pd

In [2]: test_val = pd.NaT

In [3]: from cPickle import loads, dumps

In [4]: loads(dumps(test_val))
```

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-8e4233b9b57a> in <module>()
----> 1 loads(dumps(pd.NaT))

TypeError: ('__new__() takes exactly one argument (2 given)', <class 'pandas.tslib.NaTType'>, ('\x00\x01\xff\xff\x00\x00\x00\x00\x00\x00',))
```

For the more common usage case

`loads(dumps(pd.to_datetime(['2013-01-01', pd.NaT, '2014-01-06'])))`

works as expected.
