Initial commit of what was test of converting audio format within the Audio Graph.
Initial add of mixable queue entry object
IOS-111: Progress on the playback for fading between queued items. WIP - need to fix the playback issues; I suspect audio graph needs alteration.
IOS-111: Playback now sounds correct. Next steps: 1- Fix the bytes per frame value, as we're currently setting this to 4 in the render callback. 2- Experiment with audio graph format settings and converters to see what is actually necessary and what isn't. 3- Get proper buffer cycling going so we can play the full track. 4- Introduce queue fading. 5- If blocking download thread causes timeout, implement fix explained in comment at top of STKMixableQueueEntry.m. 6- Lots of tidy up and see how we can reduce/remove duplicate code within StreamingKit. 7- Integrate the tidied STK with fading queue items into Avocado in earnest.