Wrap within requirejs module and add SimperiumModel.

Adds Backbone.SimperiumModel which works as a Backbone.Model with a Simperium
backend.

Also wraps the code in an AMD requirejs module. It will still work in the global
namespace if requirejs is not available.

When using as a module, note that it does require the Simperium js library. That
is not included here as a requirement because that library isn't yet
AMD-enabled. It therefore must be enqueued before loading this module via
require.