Add user controller with #show and #new (signup)

Resolves #11. Add a basic form that allows user creation. No sessions yet.
This test could be brittle because html ids and classes sometimes change pretty rapidly. It's kind of a bummer to have to go through and fix a ton of tests when someone touches up the UI.

I would test for the presence of the error message instead. Testing against error messages is also somewhat brittle, but probably less so than just an element id.