ENH: support for nanosecond time in offset and period
closes #1812

This pull request fixes handling of nanosecond times in periods and offsets. In addition it simplifies the internal processing of intraday periods.

``` python
In []: pd.date_range('2013-01-01', periods=5, freq=pd.offsets.Nano(5))
Out[]: 
<class 'pandas.tseries.index.DatetimeIndex'>
[2013-01-01 00:00:00, ..., 2013-01-01 00:00:00.000000020]
Length: 5, Freq: 5N, Timezone: None
```

It requires Numpy 1.7 to work, using Numpy 1.6 yields the original behaviour before this PR.

This is the continuation of #2555 after it was merged without effect fixing issue #1812.

The biggest part is a refactoring of pandas/src/period.c to use a conversion factor matrix for intraday conversion. This minimizes the impact of adding three more intraday time units (ms, us and ns). Thus the amount of frequency conversion methods is reduced to one per intraday frequency source or target. The intraday conversion factor is looked up in the conversion factor matrix inside the method.

Nanosecond support is only available when using numpy >= 1.7. When using with numpy < 1.7 everything except nanosecond support should work as expected.
