TST: Run 2.7 and 3.3 tests against OpenPyXL 2.0.2
TST: Make OpenpyxlTests version-aware

Start where it hurts most, with test_to_excel_styleconverter.
ENH: _convert_to_style per openpyxl major version

At class definition time, set _OpenpyxlWriter._convert_to_style
to whichever implementation best matches the major version of the
installed openpyxl. As of this patch, we know that we need different
implementations between v1 and v2, but we allow for the possibility
that v3 won't break this, and count on
`OpenpyxlTests.test_to_excel_styleconverter` to tell us when a new
version comes along that needs testing.