Persistent Rails assets

This will allow us to permanently store assets in the `$OPENSHIFT_DATA_DIRECTORY`. The `turbo-sprockets-rails3` gem intelligently compiles only changed assets.

The initial push will still take the normal amount of time, but subsequent pushes are much faster (obviously this is dependent on how many assets have changed).

Here is a simple experiment with the stock Rails app. Even with a small number of assets, the time was cut in half.

```
> rake assets:clean
> time rake assets:precompile

real    0m39.296s
user    0m10.213s
sys     0m1.498s

> time rake assets:precompile
real    0m19.169s
user    0m4.634s
sys     0m1.081s
```