more momery efficient describe() for DataFrame
reindex() in describe() in DataFrame was making copy of the data. I changed describe() so it processes one column at a time instead of get all columns with reindex(). It seems to be a better implementation (much less memory usage).

hi Joon, thanks, cherry-picked this into master. in the future can you make pull requests out of a feature branch? if you try to pull/merge wesm/master now you are going to generate a merge commit which will makes things a bit hairy. you may wish to git reset --hard to a commit before your change, then pull wesm/master
 Of course. Thank you!
