to_datetime parsing bug when using format
``` python
import datetime as dt
import pandas as pd
val = '01-Apr-2011 00:00:01.978'
print 'pandas version:',pd.__version__
print 'Value to parse:',val
format = '%d-%b-%Y %H:%M:%S.%f'
print 'datetime.strptime        :',dt.datetime.strptime(val, format)
print 'to_datetime, w/out format:',pd.to_datetime(val)
print 'to_datetime, w/ format   :', pd.to_datetime(val, format=format)

pandas version: 0.12.0.dev-1101391
Value to parse: 01-Apr-2011 00:00:01.978
datetime.strptime        : 2011-04-01 00:00:01.978000
to_datetime, w/out format: 2011-04-01 00:00:01.978000
to_datetime, w/ format   : 2011-03-31 23:24:13.516352
```
