Add gmean and hmean to Pandas
Currently pandas has the regular `mean` that can be called on a `DataFrame` or a `Series`, but does not have `gmean` or `hmean`:

http://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.mstats.gmean.html
http://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.mstats.hmean.html
