ENH: #3335 Pivot table support for setting name of margins column.
closes #3335.

Adds margin_column parameter to pivot_table so that user can set it to
something other than 'All'.
Raises ValueError exception if there is a conflict between the value of
margin_column and one of the other values appearing in the indices of
the pivot table.

take into account notes from closed pull request: #10296

pls also add a whatsnew note (API changes)
 done
 you have to do `message {0}".format('blah')` so 2.6 doesn't complain
 .format() change made, waiting on Travis.
 Travis tests green ;)
 thanks!
