use socket.io to optionally push data to clients

[avuserow]

socket.io is really cool and there is a workable Perl/PSGI implementation that only sucks somewhat.

socket.io would let us broadcast volume, song, and queue information to the clients in realtime. I want to make this optional given that it requires server-specific support, might eat up many connections, and we still need a trivial way for non-browser clients to consume API data. This would be for broadcasting client data only, and sending data would still happen with XHRs.

[avuserow]

Proof of concept is checked in. Not happy with the Perl module that implements this at all. I have another project using socket.io, so I'll see if I can resolve some of the flakyness.