BUG: Timestamp from "y-m-d H" string
I have datetime strings of the form `"1996-4-7 0"`, where the `0` is supposed to be equivalent to `00:00` (i.e. hour 0). I want to turn them into timestamps, but this causes ValueErrors:

``` python
pd.Timestamp('1996-4-7 0')    # Result: ValueError
```

These work, on the other hand:

``` python
pd.Timestamp('1996-4-7 00')   # Result: Timestamp('1996-04-07 00:00:00')
pd.Timestamp('1996-4-7')      # Result: Timestamp('1996-04-07 00:00:00')
```

Maybe this is intentional behavior but looks like a bug to me, or at least an inconsistency. Note the same happens for any single-digit hour, but not for two-digit hours:

``` python
pd.Timestamp('1996-4-7 8')     # Result: ValueError
pd.Timestamp('1996-4-7 11')    # Result: Timestamp('1996-04-07 11:00:00')
```

Problem occurs in both stable 0.15.2 and 0.15.2-163-g671b384.
