.describe() for series of objects
See #210.

I don't know if `series.dtype == object` is the best way to check for non-numeric data, but it appears to work.

At present, I've used fields 'count', 'unique', 'top' (i.e. most common) and 'freq' (i.e. count of the most common). These seem like obvious things for a summary.

I also noticed a problem doing `df.describe()` if df has integer columns, so I copied some code from series to fix that.
