Server api to log playback

[rubbersoul]

It would be nice for apps to be able to tell the server a song is playing so that recently played list stays up-to-date. Right now it seems that the server assumes downloads are plays and there is no manual way to tell the server that a user is playing a cached song from a subsonic client.

Use case is for mobile clients where caching of music is common and many plays may come from that cached list and never get logged on the server side.