change apps:add to apps:register

This more accurately reflects the operation, since the facebook
app already exists - we're just registering it in the local .fbturc.
change users:add to users:create

This more accurately reflects the operation, since we're actually
creating a new user.  It also makes space for an 'add' operation
which adds an existing user to an existing app.
cli.rb: change find_app! to module method

This will allow it to be reused by FacebookTestUsers::CLI::App.
support adding existing users to other (existing) apps