Add Precompile assets step

Here's one more (optional) step which I need, as I use the [asset_sync gem](https://github.com/rumblelabs/asset_sync).
I can't use the heroku labs feature `user_env_compile`, as this interferes in my case with the [heroku-buildpack-multi](https://github.com/ddollar/heroku-buildpack-multi) I'm using.

Thus, asset precompilation fails when pushing the app https://github.com/rumblelabs/asset_sync#fail-silently and I need to run precompile asset as a separate step.

I would be glad, if you could merge this, so that I don't have to maintain a fork.