Watch for asset changes.

This paver command creates a watchdog that monitors changes to the coffeescript and sass files. It's enabled by default.

If you use devstack and do your editing in the host OS, the monitor won't see your changes (inotify doesn't work across the NFS link). But you can run `paver watch_assets` in the host OS. On OS X, you'll have to upgrade sass (`sudo gem install sass`), and `pip install paver psutil watchdog`.

@singingwolfboy @cpennington @wedaly