Reference Rails::Engine::Railties within Engine

Referencing Rails::Application::Railties leads to an exception when running under JRuby

Fixes #169
tab through login elements in the order they appear

Fixes #171
Correct admin login title in user_sessions/new
Adding support to format js when creating a new session

Fixes #173
New user session form is now being rendered directly from checkout registration template instead of being called from override

Fixes #175
Fix logout links

Fixes #182

Conflicts:
	lib/views/backend/spree/layouts/admin/_login_nav.html.erb
Remove loose require for coffee_script

Fixes #176
updated the UserRegistrationsController#create action to respect Devise after_sign_up_path_for

Fixes #183
Fixes #181

Conflicts:
	spree_auth_devise.gemspec
Use font-awesome icons in admin/_login_nav
Add autofocus to login form

Springing from the discussion in https://github.com/spree/spree/issues/4715, this change allows the HTML5 `autofocus` attribute to put the cursor into the email field when displaying the login screen, which removes one step from the user login process.

Closes #190
Redirect to login page if the user is not logged in.
Fixes #177