ENH: microphone recording: enable selection of stereo or mono
ENH: microphone.getRMS() returns "loudness" of audio

- from .wav file or from array (1D, 2D)
- use to give subjects feedback about whether they are talking loud enough
RF: minor clean-up in microphone and sound.py
RF: BufferImageStim accepts mask and pos params

This allows dynamic grab of a small screen area w/ mask, e.g., for
gaze-contingent displays.
Merge pull request #331 from jeremygray/master

ENH minor enhancements, RF's