BUG: groupby segfaults when passed a function of a timestamp which raises a TypeError
It _is_ a dumb thing to do, but pandas probably shouldn't segfault regardless:

``` python
In [1]: dates = pd.date_range('01-Jan-2013', periods=12, freq='MS')
   ...: ts = pd.TimeSeries(randn(12), index=dates)

In [2]: ts.groupby(lambda key: key[0:4]).first()
It seems the kernel died unexpectedly. Use 'Restart kernel' to continue using this console.
```

Tested with pandas 10.1 on  win32/win64,  Python 2.7 and pandas 0.11.0.dev-2f7b0e4 on win32, Python 2.7.
