Moves devise based user model from core into its own gem.

Now you can provide your own user model and integrate it into Alchemy.

Add a initializer into config/initializers:

~~~
#config/initializers/alchemy.rb

Alchemy.user_class_name = 'YourUserClassName'
Alchemy.login_path = '/your/login/path'
Alchemy.logout_path = '/your/logout/path'
~~~

In order to get the `Alchemy::User` back into your app simply add the `alchemy-devise` gem into your `Gemfile`.

~~~
#Gemfile

gem 'alchemy-devise', github: 'magiclabs/alchemy-devise'