fix subcommands (issue #1)

facebook_test_users/cli.rb was incorrectly defining subcommands
via Thor by nesting classes.  This resulted in subcommand tasks
appearing in the top-level help, which was very misleading.
rename misleading 'new_guy' variable to 'new_app'
remove the default 'list' task for the 'apps' subcommand

Ideally we would keep 'list' as the default task for the 'apps'
subcommand, but right now, this bug in thor means that default_task
breaks subcommand help:

  https://github.com/wycats/thor/issues/306

so it's probably better to leave it out for now.
add missing json dependency to ensure to_json is available
add spec for adding user to non-existent app
add --name, --locale, and --installed to user creation
add ability to change name/password of an existing user