PR: Add Convolver module

This PR adds a Convolver module to webaudiox.  I added the following:
webaudiox.convolver.js - the module
ConvolverDemo.html - demo 
telephone.wav, spring.wav, bright-hall.wav, and muffler.wav to the examples/sounds folder
updated webaudiox.js to include/concatenate the new convolver module with the rest
updated readme.md to reflect new module and usage info