Added `Workbook.current()` class method
Modified `Workbook` constructor to check if `fullname` is a COM object, if so build Workbook from it
Added `is_xl_object` to both platforms to check if the argument is a COM object (on Mac it just returns False)
Added mechanism to specify Excel instance on which `new_workbook` and `open_workbook` should act.
The changes should not affect the Mac version
Also tiny bug fix: expression `len(sys.argv) >= 2 and sys.argv[2] == 'from_xl' should have strict inequality.
Merge branch 'ericremoreynolds-master' into develop