Null audio backend #39

Provide a dummy audio backend that syncs to the system high-resolution clock but otherwise does not require any drivers present. This will be used as a last-resort when the running machine has no audio hardware and software at all.

Note that this is different from #38, the Null backend is not synced at all and can run as fast as possible. It can also use existing audio data for inputs and write to output buffers.

EDIT: Renamed Dummy to Null (confusingly) because I think that's a better name and is the name I ended up using in #111.