BUG: to_datetime returns KeyError: 'p' when %p specified in format string
Here's a question on SO - http://stackoverflow.com/questions/19635704/keyerror-in-pandas-to-datetime-using-custom-format, also possible to check it like this:

```
pd.to_datetime('09/30/2013 04:14 PM', format="%m/%d/%Y %I:%M %p")
```

It works for python time:

```
time.strptime('09/30/2013 04:14 PM', "%m/%d/%Y %I:%M %p")
```
