added describe and scoreatpercentile in frame.py
Added two methods in frame.py: describe, which returns a DataFrame with summary statistics of each columns of the original DataFrame (with index=columns), and scoreatpercentile, which calculate the score at the given percentile.

When I play around with a dataset I use this describe thing a lot so I thought it would be convenient to have this with DataFrame.

I think this is crudely implemented so if you think there is a better way please let me know!

-Joon
