.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.

this looks good and thanks for the min/max bugfix. will need to add a compatability function for Counter which is not present in Python 2.5 which I'm still supporting
 Actually, checking the docs, 2.6 doesn't have Counter either. It's pure Python, so the simplest thing is probably to copy the 2.7 version and ship it in a module in util. Let me know if you want me to do that.
 would you please? separate pull request since I already rebased and merged this one
 OK, no worries.
 Do you have an opinion on how NA (NaN/None) should be handled in the output of Series.describe? I should also take care with that in value_counts
