ENH: allow layout for grouped DataFrame histograms
Also remove the slow decorator for tests that take < 100 ms time

closes #4102.

closes #4194.

@jreback any comments b4 i merge? nothing drastic here just supporting the new layout argument with the by argument
 oh release notes
 i think this could be merged once it passes...nothing super drastic, just allows the layout keyword to work when the by keyword is passed
 eh i'll wait...not critical
 @jreback shoudl this be marked for 0.12 to close the kde plot issue? or i can mark that one for 0.13...even though it is probably fixed, if we do 0.12.1 it could go there instead as a bug fix along with this...
 up2u
 up2u
 k i'll mark #4194 for 0.13/0.12.1, just want to get those html bugs squashed!
 @jreback anything to add here b4 i merge?
 ok
