Don't watch any files outside the app root

[jonleighton]

This seemed like a nice idea at the time but it means we're watching lots more files than just those under the app root, so using more CPU and I think it's simpler to just say "spring will only notice changes under your app root". If you change e.g. a gem which resides on the filesystem then you'll need to stop spring to notice the changes.

[rwjblue]

This could be wrapped up in the changes to ApplicationWatcher to add polling.

Most of the external polling libraries (rb-inotify and rb-fsevent) only support watching directories as far as I can tell.