Rename service_recording to recording in README

It was hard to tell where service_recording was coming from in the
playback examples. Having it align with the recording example shows
folks makes it more obvious what's being passed to replay.
replay_recording can take a recording object

It seemed weird to have to know that you need to call to_h on a
recording to get it to replay. This makes the readme example tell a more
cohesive story.

Also slight cleanup on a redundent or
Add instructions on replaying persisted recordings
Merge remote-tracking branch 'twinn/replaying_recordings'
Clarify replayable operation test

 * Removed the test covering passing in a hash, it was unnecessary
 * Added comments to help describe the story