Include paths in track info and filter params

If this API is going to be the glue between music players and libraries, then it needs to let you retrieve and filter by path info.

I'm working on [a music web app](https://github.com/irskep/summertunes) and would love to speak this API and be compatible with multiple things. But without the path in the track info, there is no way for me to add any tracks to the playback queue! And without track querying by path, there is no way to show the "now playing" area based on the currently playing file reported by the player.