Add layout keyword to dataframe.hist()
closes #4050. The current version defaults to a very sane default, but sometimes for presentation I prefer them all to be all in a row, or all in a column, or some non-balanced mix of the two.  This keyword gives the flexibility for all of those options.

is this a 'standard' option name? it seems intuitive, but does mp use this in a similar context (or some other keyword)? just asking, seems logical
 fyi..in future, pls create an issue, then reference it in the PR, we can label issues so categorizing is easier..thanks!
 Sorry about that, was unaware about the issue preference.  Made issue #4050, added issue to release notes and original commit message.

As for the name, I just copied the name/interface of "layout" from plotting.grouped_hist(...).  I don't know too much about the collision with matplotlib, other than when matplotlib is talking about layouts they are referring to the same type of thing, and it doesn't appear to me that matplotlib actively uses this keyword.  However, I am perfectly happy to change it, I have no strong connections to this name.

I think perhaps the best alternative would be to break this into two keywords: rows/cols?
 this looks good
can you hook up Travis?
then recommit?

git commit --amend -C HEAD
 Alright, Travis is hooked up.  One particular unrelated test is a bit flakey, but otherwise the tests are all passing: https://travis-ci.org/danbirken/pandas
 @danbirken flaky tests should be resolved...can u rebase and push again? thx
 There we go.  Still had a transient failure, but after resetting that test, we are fully green.
 @danbirken thanks much!
