Enable Simple signup

This is just a very simple user creation addition.
There is now a link on the default page to take you to signup.
I posted my questions throughout the code.

@melari, signup works, but I created this PR mainly to get some feedback.
The current user object should actually be already available in `@current_user`, rather than loading it from the database a second time. (see https://github.com/foodly/foodly/blob/master/app/controllers/application_controller.rb#L7)