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.
Show Errors as popup on Mac, closes issue #57 and closes issue #51
Implmented Range.autofit(), closes #33
Merge branch 'ericremoreynolds-master' into develop
made Workbook.current() cross-platform compatible
reimplemented _xlwindows._get_latest_app
require minimum version of psutil, closes #48
made atleast_2d work for scalars, closes #53
fixed _get_latest_app in case Excel has been closed
account for Excel 2010 array formatting bug (#43)
Added sheetname_only parameter back in
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	xlwings/_xlwindows.py
	xlwings/main.py
Revert "Added sheetname_only parameter back in"

This reverts commit 94adf6ef2c20065ed03df9e40e3c0664e122df8a.