Git or mount

Added a ``github_or_mount()`` to ``get_webwork`` subroutine.  This function is called twice, before ``get_webwork2_repo()`` and before ``get_pg_repo()``.  It asks the user to decide wether to mount the appropriate directory or to obtain it from github.  If the user chooses to mount, then the appropriate file must be available for mounting through virtualbox.  That is, the user must have already made the folder available for mounting via settings -> shared folders on the guest machine.  The files are mounted with ownership ``www-data:wwdata``.  This is so to allow serve to write without permission errors.  There was no way that I could find to mount and change ownership to target precisely the files that need to be writable by the server.