Optional execution on precompile

We love asset_sync, but we don't like that it bundles itself into the assets:precompile task. Our deploy system is such that this task can be run on multiple machines simultaneously, and we only want one box to upload its results.

I've created an `assets:sync` rake task which performs synchronization, and added a `run_on_precompile` configuration setting which, if true (default is true), will run that rake task after the assets:precompile rake task.

The default behavior remains exactly the same, but this provides a way for people to disable the automatic `assets:precompile` rake enhancement.