Start to move command line processing to a different module

Adding the 'twitter-auth' command to extras.py caused a circular import
to enable till.command to be used in extras.  Move the command class to
a separate module to avoid this.  In the future the various commands can
be moved to more appropriate modules, eg. user.py for 'adduser'.
Twitter auth using python-requests-oauthlib

Working towards completion of issue #58 on github.
Move adduser command implementation to user module
quicktill 0.10.44