BUG: let distribution methods raise an error if too many args given.

Closes #1815.
DOC: document rvs() method of distributions in stats module better.
MAINT: in stats.distributions, let rvs method accept "size" as positional arg.
BUG: handle correctly situation with 2 args too many in stats.distributions.
Merge pull request #400 from rgommers/stats-ticket-1815

BUG: let distribution methods raise an error if too many args given.