PERF: infer_datetime_format fails without zero-padding
I think this can be fixed by just padding tokens in the inference step.  The dates _do_ parse, just always fall back to the slow path, even with `infer_datetime_format=True`

```
In [22]: from pandas.tseries.tools import _guess_datetime_format

In [23]: _guess_datetime_format('02/02/2014')
Out[23]: '%m/%d/%Y'

In [24]: _guess_datetime_format('2/2/2014')
```
