Spree 2.1 compatibility.

Key changes:

1. Replace Rails 3 mass assignment with strong parameters
2. Update gemspec and Gemfile to reference unreleased 2.1.0.beta
3. Update JS and CSS files to refer to the broken out source files (spree_frontend, spree_backend) instead of spree_core
4. Remove unused model class
5. Reorganize engine class into a dedicated file, use include in lib/spree_store_credits.
6. Change translations to use Spree.t
7. Update FactoryGirl syntax
8. Add a number of specs
9. Update existing specs for compatibility with Spree 2.1 changes
10. Move spec/requests to spec/features
11. Add simplecov
12. Use database_cleaner for database cleanup
13. Update .travis.yml to run against all branches, remove notifications, and add xvfb