Redo styles, based on Metis repo
User model knows which other Users are followers
Users can follow and unfollow other users

* Add helper methods to User for following and unfollowing
* Add helper method to User to determine if a user follows another user
* Display follow/unfollow button on users#show
* Add routes to create/destroy FollowingRelationships
* Add FollowingRelationships controller w/ create and destroy methods
* Allow non-alpha characters in :id for users/
Styling for buttons
Minor styling tweaks
Show message for no followers