Use url helper instead of raw url generated based on the site_url spree preference
Updates to make code decoupled from devise.

Conflicts:
	Gemfile
Fix user references to spree_user.
Fix remaining references to :user noticed by @swrobel.
Attempt fallback to social authentication for user.
Add spree engine name in the omniauth auth url helper
Merge pull request #78 from supherman/url_helper

Use url helper instead of raw url generated based on the site_url spree ...
Replace @spree_user with @user because spree_auth_devise controllers still use @user