File transfers can now be reversed with the --receive flag when launching onionshare.py.  The receive_mode.html now has variables that allows it to be reused when the user sends the file.  The `except Exception as e` in `receive_file()` is just there in case I need to debug later.  This mode is not yet accessable with the GUI (though that is getting reworked so I don't know if I should try to get it to work with the current GUI) and there is no progress bar, but it is a good start for getting #97 done.