Convert into a Rails Engine

[obie]

• should be able to use magma-chat as a dependency of another Rails project (aka Rails Engine-style gem)
• don't kill the ability to run magma-chat as a standalone app the way it works now
• make the inclusion of magma-chat's User class optional (default: false) so that it doesn't clash with host app User
• make the inclusion of magma-chat's session and authentication optional too
• change the current application layout to be specific to the chats controller so it doesn't clash with host app