availability of nanosecond periods in DatetimeIndex
Hi there,

when trying to create a DatetimeIndex for data sampled in the nanosecond range

``` python
pd.DatetimeIndex(start=pd.Timestamp('2012-12-01'), periods=10, freq=pd.offsets.Nano(2800))
```

there appears an error due to an uninitialized timestamp.

Inside the pandas code python datetime objects seem to be used for time representation of "slower" than nanosecond times, whereas the nanosecond period seems to appear as a simple int objects which cannot be handled by the internal arithmetics at the moment.

Is there a plan to switch to nanosecond aware time objects internally? As I would like to use this great library for fast data as well I can also offer to help working on that.

Best regards
Andreas
