Dynamically require listeners.

Change listeners require from `lib/ryespy.rb` => `lib/ryespy/app.rb`
`App#setup`, to support future conditional dependencies. Simplify
`App#refresh_loop` to simplify `@config.listener` dispatch.
Implement Listener::RaxCF , for Rackspace Cloud Files.

The `RaxCF` listener listens to one or more file key prefix filters in
a Rackspace Cloud Files container, looking for new or changed files.

`README` and `ryespy.gemspec` are updated to document usage and the
extended utility of Ryespy.

This listener is fully functional.