Spree 2.1 compatibility.  Removed mass assignment and added strong parameters.  Added required JS and CSS files.  Minor refactoring.  Fixed a number of controller and helper issues, to ensure resources are loaded and responses redirect as designed.  Added a number of specs.  Added simplecov support.  Changed file link to return a 404 if the link is unavailable.
Merge pull request #43 from halo/feature/spree_2_1_support

Spree 2.1 compatibility - merging after leaving the PR open for a weekend for comments/questions/concerns.