Finishes audio control refactoring

Closes #110
Makes vector mode optional

This commit merges the two RT audio implementations, i.e.:

	- The older single-step implementation
	- The new vector implementation

The implementations are determined at compile time and are selected using the
kVectorMode flag in "signal.h". 0 means the old implementation, 1 means the new (vectorized) one.
Notice that NRT processing always use single-step.

Both support basic audio (sines etc), mixing, control values. Things *not* supported
in the vector implementation include loops, delays, counter and rec/play (which depend
on counter). Custom processors must include "signal.h" and check for the kVectorMode flag,
the AU wrapper have been updated to do this.

See #111

Issue remains open until all features are ported to the new implementation.
Updates test