ENH: Make ExcelWriter & ExcelFile contextmanagers

both can be used in with statements now. Makes it easier to use with
multiple writers, etc.

Plus touch up the docs on writers and such.
Merge pull request #4933 from jtratner/make-ExcelWriter-contextmanager

ENH: Make ExcelWriter & ExcelFile contextmanagers
ENH: allow hist to take a string "by" argument