Add DataFrame with archive summary info

[cdeil]

Let's add a convenience attribute or method to get a pandas.DataFrame that summarises some stats for each archive.

This:

>>> wsp.meta.archives
[WhisperArchiveMeta(index=0, offset=52, seconds_per_point=10, points=1555200, retention=15552000),
 WhisperArchiveMeta(index=1, offset=18662452, seconds_per_point=60, points=5256000, retention=315360000),
 WhisperArchiveMeta(index=2, offset=81734452, seconds_per_point=3600, points=87601, retention=315363600)]

But it would be nice to also get the data summary, especially timestamp of first and last data point, actual number of data points (non-zero) in each archive, maybe also min/max/mean of values?