Using wider range of bitrate in rd_collect
Parallelize vq_train and some minor restructuring.

This uses openmp to do a quick parallel implementation of the VQ
 training inner-loop.

It can be flipped to uni-processor with a simple flag in the source
 for cases where multiple trainings will be run in parallel, since
 thats somewhat more efficient than the multithreaded version here.

r=jm