Timestamp losing nanosecond accuracy when reading from string
Hi,

I'm on pandas 0.14.1. Timestamp seems to lose nanoseconds when converting to string and back. Wonder if this is bug.

``` python

In [18]: pd.Timestamp('20010101 00:00:00')+pd.tseries.offsets.Nano()
Out[18]: Timestamp('2001-01-01 00:00:00.000000001')

In [19]: pd.Timestamp('2001-01-01 00:00:00.000000001')
Out[19]: Timestamp('2001-01-01 00:00:00')
```

Thanks,
Mark
## INSTALLED VERSIONS

commit: None
python: 2.7.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.11.0-26-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.14.1
nose: 1.3.0
Cython: 0.19.2
numpy: 1.8.1
scipy: 0.13.2
statsmodels: 0.5.0
IPython: 0.13.2
sphinx: None
patsy: 0.2.1
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.2
bottleneck: 0.8.0
tables: 3.0.0
numexpr: 2.3
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.8
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
