Mozilla Bug 899050

Add rudimentary support for multichannel files when using WASAPI.
Mozilla Bug 907817

Actually set the latency when using the audiounit cubeb backend.
Mozilla Bug 907817

Add a cubeb API to get a valid audio latency range per platform.
Mozilla Bug 918861

Add an API to get the native samplerate for a given audio backend.
Merge pull request #35 from padenot/master

Sync back gecko changes