Adding devise

Added Devise per the instructions in [User Authentication](https://www.bloc.io/users/joe-palmieri/checkpoints/308) checkpoint. 

I have the root going to welcome#index, but I think I'd want it to actually root to the user's todo list (like [Trello](http://www.trello.com) does), and authenticate the user, so that it redirects to the login flow in the case the user isn't signed in. Hopefully this won't be too hard to do after-the-fact.

Sorry for this giant pull request. I didn't realize that branching from an unmerged branch and creating a pull request would show a diff in relation to origin/master. Should I always branch from master when creating a branch for a pull request?