added basic user authorization

@RichardBlair 
Updated routing, controllers, and views in adding user authentication to dickbook to match Rails 4.1.1 standards utilizing information from the following resources:

http://railscasts.com/episodes/250-authentication-from-scratch
https://github.com/rails/strong_parameters
and, of course, advice from https://github.com/RichardBlair
You need to write proper tests for this.

So, you need to test that posting to UserController#create does indeed create a new user. You can use `assert_difference` for this.