Metadata - Add audiotrack ID, playlist length and asset complete.

You mentioned the lack of an "asset complete" metadata update in #192. I wanted to implement that originally, but forgot. Having the image data stay up afterwards should be optional for the dev/user/etc. I've added everything I can think of to the metadata. Now it goes like this:

* stream_started=true
* audiotrack=1&playlist=0 (When nothing is in range)
* audiotrack=1&playlist=5&asset=2&tags=4,3,2 (When in range and a new asset is starting to play)
* audiotrack=1&playlist=5&asset=2&complete=true (When the current asset is complete)
* audiotrack=1&playlist=4&asset=4&tags=4,3,1
* audiotrack=1&playlist=4&asset=4&complete=true

Plus, this supports multiple audio tracks :)