Set up initial method for receiving and saving files.  this only works for windows right now (I think)
Some changes with strings.json are to counteract what I did in master (Why would I ever develop in master?) Added a new HTML file for when in receive mode and changed `filename` arg to `--filename`.  This was to make `--receive` and `--filename` mutually exclusive.
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.
added the ability to designate where the file will be saved
Submit for now allows for the sender to add the SHA1 hash if they pre-generated it.  This information is passed along to the receiver in the console.