TST: Cleanup Excel tests to make it easier to add and test additional writers
DOC: v0.13.0.txt cleanup
Merge pull request #4844 from jtratner/cleanup-excel-tests

TST: Cleanup Excel tests to make it easier to add and test additional writers
ENH: Added xlsxwriter as an ExcelWriter option.

Added xlsxwriter as an optional writer engine. Issue #4542.
CLN: Using new excel test class for test cases.
Merge branch 'enh_xlsxwriter_refactor' of github.com:jmcnamara/pandas into enh_xlsxwriter_refactor