ENH: Added xlsxwriter as an ExcelWriter option.

Added xlsxwriter as an optional writer engine. Issue #4542.
Merge pull request #4857 from jmcnamara/enh_xlsxwriter2

ENH: Added xlsxwriter as an ExcelWriter option.
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.