excel, build: Pin xlwings wersion to 0.2.3 (0.3.0 broken!!).

- excel: Add utility funcs to open or get current Workbook, when run from python.
-- FIX not to add empty sheets when overwritting existing ones.
- docs: CHANGES mention broken behavior on speed-down to a halt.
-- README improve project's coordinates on landing page.
- ci: FIX py27 failing on setup.py reading ver with unicode.