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
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.