User-Idea relationships, API token authentication, refactoring

I changed the User-Idea relationship to now be a one-to-many relationship, using a migration which, when run, should convert all our existing relationships between the two models to this new one. BC, when you push this to Heroku, please be sure to run `rake db:migrate` to ensure my changes get pushed to the Heroku databse.

I also added preliminary API token authentication, since I figure that we only want to allow API requests from our own Android app. Out of respect for the Android team, I haven't flipped the switch on this yet, but we should figure out a way to create a token which isn't accessible or viewable on GitHub (because our github repos are public). For now, this code is commented out, but once we make the app available it'll definitely need to be enabled.

Many of the changes in this pull request are also just simple refactorings, making the code clearer and more concise.