Pandas 0.13 bug parsing am/pm in time stamp strings.
Here's an example of the problem:

``` python
In [16]: pd.to_datetime("12/01/13", format="%m/%d/%y")
Out[16]: Timestamp('2013-12-01 00:00:00', tz=None)

In [17]: pd.to_datetime("12/01/13 3:15", format="%m/%d/%y %I:%M")
Out[17]: Timestamp('2013-12-01 03:15:00', tz=None)

In [18]: pd.to_datetime("12/01/13 3:15pm", format="%m/%d/%y %I:%M%p")
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-18-a00cce7186a4> in <module>()
----> 1 pd.to_datetime("12/01/13 3:15pm", format="%m/%d/%y %I:%M%p")

/home/ely/anaconda/lib/python2.7/site-packages/pandas/tseries/tools.pyc in to_datetime(arg, errors, dayfirst, utc, box, format, coerce, unit)
    139         return _convert_listlike(arg, box=box)
    140 
--> 141     return _convert_listlike(np.array([ arg ]), box=box)[0]
    142 
    143 class DateParseError(ValueError):

/home/ely/anaconda/lib/python2.7/site-packages/pandas/tseries/tools.pyc in _convert_listlike(arg, box)
    113                 # fallback
    114                 if result is None:
--> 115                     result = tslib.array_strptime(arg, format, coerce=coerce)
    116             else:
    117                 result = tslib.array_to_datetime(arg, raise_=errors == 'raise',

/home/ely/anaconda/lib/python2.7/site-packages/pandas/tslib.so in pandas.tslib.array_strptime (pandas/tslib.c:20989)()

KeyError: 'p'
```
