Render labels in Devise views just for screen readers

Based on concepts published on
http://webaim.org/techniques/css/invisiblecontent/
Remove meta programming defining omniauth callbacks

Using #action_missing as a fallback to get existing OauthProviders.
Clean Omniauth controller using #action_missing to define callbacks
Create UserInitializer to generate new users via Omniauth
Move UserInitializer class to its own file
Store new authorizations without email by default
Attach orphan omniauth authorizations after login
Create OmniauthUserSerializer
Use OmniauthUserSerializer instance to sign users via omniauth
Accept authorizations attributes nested in User model

The validation for presence was removed because we already have it in
the database. Having it here causes problems with Rails' #accepts_nested_attributes_for.
Revalidate omniauth data after user filling his missing e-mail
Use OmniauthSignIn class to update user's attributes on login
Define status for an OmniauthSignin
Minor style adequations
Remove not used parameters from OmniauthSignin interface
Define access tokens methods to omniauth user serializer
Allow setting email in the omniauth signup process
Define social networks urls on omniauth user serializer
Move ability to setup an omniauth sign to a dedicated method
Permit setting sessions/new to receive :email param to render on view