Loss of nanosecond resolution when constructing Timestamps from str
Looks like a bug: When passing a YYYYMMDDTHHMMSS.f string to `pd.Timestamp`, the resolution seems limited to micro- rather than nano-seconds:

``` python
>>> pd.Timestamp('20130101T000000.000001+0000')
Timestamp('2013-01-01 00:00:00.000001+0000', tz='UTC')

>>> pd.Timestamp('20130101T000000.0000001+0000')
Timestamp('2013-01-01 00:00:00+0000', tz='UTC')

>>> pd.Timestamp('20130101T000000.00000001+0000')
Timestamp('2013-01-01 00:00:00+0000', tz='UTC')
```

The behavior seems correct when using a different string format:

``` python
>>> pd.Timestamp('2013-01-01 00:00:00.00000001+0000')
Timestamp('2013-01-01 00:00:00.000000010+0000', tz='UTC')
```

Using pandas 0.16.0, python 2.7.6
