Add volume amplification (a.k.a. dynamic range compression) to pulseaudio backend

Currently the pulseaudio backend doesn't support volume amplification (or dynamic range compression).  This patch fixes that.

I have tested the patch and changing the volume amplification in the gui does now work with the pulseaudio backend, but there are still a couple of things that should be looked at:
- I'm currently allocating a new buffer, copying the current data to the new buffer, changing it, sending it to pulseaudio, and then freeing it.  Attempting to modify the data in-place is causing a segfault and I'm not sure why.
- I've copied the #ifdef __SSE___ line directly from the CoreAudio backend; it is untested.