Fix asset path munging to work in production env.

Fix issue #31 by changing the initializer method. If you use `after_initialize` you can't then change the Sprockets path in production mode, as it becomes immutable.

Also, clean up the way the variant configs are initialized to fix issue #11.

I pulled out these specific changes from my server-rendering branch, so that they do not depend at all on server-rendering. I will rebase the server rendering changes on top of this so that they can be pulled in separately at the appropriate time.