Fix config.assets.cache_store not working anymore

In Rails 3.2 setting `config.assets.cache_store` would allow changing the cache store sprockets used. This feature was removed when sprockets was ported to a separate repository (for reasons unknown to me). I've added it back as well as added tests for it.

I had to `bundle exec rake test` to get the tests to pick up my changes to the railtie.

For environments without the option of a shared tmp directory, this speeds up asset compilation by 4 times in some scenarios (from 66 seconds to 16). Or on my local machine, 16 seconds to 5 (if you subtract out the 4.5 seconds it takes rails to boot on my local, that's a significant improvement: 12 seconds down to <1).  ![](http://erc.bz/xwQM.jpg)

**Edit:** By shared tmp directory I mean a persisted tmp directory across deploys (e.g. Heroku doesn't have a persisted tmp directory).