PIDVR-10: Design Egress API

There should be a second API, specifically for allowing storage devices to grab recordings from the recordings_cache.

Actual download will be via HTTP (See PIDVR-9), but some sort of API call will be needed to ascertain the filename which should be requested.

It seems that TVHeadEnd doesn't have an API per se, but that it's possible to use JSON POST requests in order to achieve the functionality normally offered through the Web interface.

A storage appliance wanting to grab a recording will need to know the following (as a minimum)

- Subdirectory to find the recording in (if any)
- Filename of the recording

Ideally, it should also be able to retrieve

- Filesize
- Series Name (and number)
- Episode Name (and number)
- If possible a checksum to aid in download verification

Once it's retrieved and verified the recording, it should be possible to trigger deletion of the file, though that will probably be implemented seperately.