Improved OmniAuth strategy support

Removed app_id and app_secret as required arguments for OmniAuth strategies.
Arguments passed directly to initialize new strategies are specified as
omniauth.providers['reqargs'], and optional arguments can still be specified
as either an Array or Hash as omniauth.providers['optargs']. This change
allows OmniAuth strategies that are not OAuth-based to be used without
changing non-config files such as config/initializers/devise.rb