Spree 2.1 compatibility

Changes for Spree 2.1 compatibility:

1. Updated the gemspec and Gemfile to point to the 2.1.0.beta version and specified the appropriate minimum Ruby version.
2. Removed mass assignment and added strong parameters.
3. Added required JS and CSS files.
4. Fixed a number of controller and helper issues, to ensure resources are loaded and responses redirect as designed.
5. Added a number of specs, especially for controllers.
6. Added simplecov support in a manner similar to Spree's simplecov integration.
7. Changed file link to return a 404 if the link is unavailable.
8. Minor refactoring to simplify methods.

@halo @iloveitaly @jdutil I'm going to leave this PR open through the weekend for comments/questions/concerns.  If everything looks good I'll merge it in on Monday morning.