Extract Listener::RaxCF to Listener::Fogable .

Move `Listener::RaxCF#check` => `Listener::Fogable#check`. Move
`Listener::RaxCF#get_unseen_files` =>
`Listener::Fogable#get_unseen_files`. Update tests.

Also change `#get_unseen_files` to ignore 0-length files.
Implement Listener::AmznS3 , for Amazon S3.

The `AmznS3` listener listens to one or more file key prefix filters in
an Amazon S3 bucket, 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.