Refactor OmniAuth integration.

- Make use of Devise's OmniAuth integration.
- Merge config/initializers/omniauth.rb into config/initializers/devise.rb.
- Define button labels in config/locales/devise.en.yml.
- Fix devise's routes so we can use user_omniauth_authorize_path().
Remove the constraints on services callback, since there is a spec against it.
Load spec/env.test on application boot.
Look up the OAuth values on Devise instead of ENV.
Add specs for icon_for_service.
Add specs for other ApplicationHelper methods while I'm here.
Merge pull request #558 from knu/refactor-omniauth

Refactor OmniAuth integration.