Implement Signals for reading buffers and controlling phase

[colinbdclark]

In order to support user-specified wavetables (for oscillators, static envelopes, etc.), we need at least two new signals:

1. Something like Flocking's readBuffer, which cycles through a buffer based on its phase input
2. One or more phasors, which either take a specified frequency (such as in CSound) or a step size and rate per step (as in SuperCollider and Flocking) and integrate a normalized phase value (i.e. 0->1) accordingly. Either way, a trigger input should enable the phase to be reset to the beginning.