ENH: allow hist to take a string "by" argument
closes #4103.

@jreback ok 2 merge? this adds the ability to specify a string column in the by argument of `hist`
 ok
 i htink i'm going to wait here until dup plotting is fixed i don't want to complicate things more than they already are
 closing because this feature shouldn't be added until the plotting API is more unified .. it's just not essential right now
