Groupby.mode() - feature request
#### Feature Request
Hi,

I use pandas a lot in my projects and I got stack with a problem of running the "mode" function (most common element) on a huge groupby object. There are few solutions available using aggregate and  scipy.stats.mode, but they are unbearably slow in comparison to e.g. groupby.mean(). Thus, I would like to make a feature request to add cytonized version of  groupby.mode() operator.

Thanks in advance!

Jan Musial
