improvements to file upload handling

[shcheklein]

• if filename is passed make sure that we always close it (open happens on our end). Related - Added a try except block at the end of the Upload() method, trying to… googlearchive/PyDrive#144.
• create an alternative SetContent to pass file descriptor. Check what is the best way to do this. Related, but not ready to merge - Update files.py googlearchive/PyDrive#166
• Write tests for both, update docs, etc.