Spree 2.1 compatibility

This PR is built on top of PR #42 , which added Spree 2.0 support.  I would suggest that PR #42 be merged to master, branched to 2-0-stable, and only then have this PR merged to master.

Key changes in this PR (above and beyond those in PR #42):

1. Replaced Rails 3 mass assignment with strong parameters
2. Updated gemspec and Gemfile to reference unreleased 2.1.0.beta
3. Updated JS and CSS files to refer to the broken out source files (spree_frontend, spree_backend) instead of spree_core
4. Removed unused model class
5. Reorganized engine class into a dedicated file, use include in lib/spree_store_credits.
6. Changed translations to use Spree.t
7. Updated FactoryGirl syntax
8. Added a number of specs
9. Updated existing specs for compatibility with Spree 2.1 changes
10. Moved spec/requests to spec/features
11. Added simplecov
12. Used database_cleaner for database cleanup
13. Updated .travis.yml to run against all branches, remove notifications, and add xvfb