Only call `spree.root_path` whene it's defined
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
Use new Store concern for current store methods.
Fix admin devise controllers for current_store as well.
Replace deprecated `Railties.engine`