Web API: data versions

Hi there,

High priority:
- I'm currently working on improving the beets integration in Tomahawk Media Player (both desktop and android versions). A feature I'd really love to see would be to have a simple revision string returned in the /stats response from the web plugin. A simple timestamp indicating the last modification would be enough. That way I would be able to only re-fetch the beets collection data when necessary and not on every start-up.

Low priority:
- Even more awesome would be if I'd be able to give that timestamp to a query like /item/diff/$timestamp and get a list of all added/modified and removed items since that given timestamp. That would obviously be more efficient and should improve performance even further :)
- Another thing that'd be real handy would be to have some way of getting the beets version through the web plugin. I'd like to make sure that every necessary feature is supported by beets before initializing the collection in Tomahawk.

Thanks!

Start by locating the web plugin entry points for /stats, /item/diff/$timestamp, and version reporting. Clarify the timestamp semantics and response formats before implementing; done should include incremental added, modified, and removed items plus a way to identify the beets version.

Written by the indexing model from the issue text.