Bandwidth api

I wanted a way to pull per-server bandwidth statistics from the API. 

There is already "history" which has the totals, but I didn't see anything that exposes the per-server stats like "/config/server".  So, I added a new API mode for "bandwidth" which has both sums and per-server stats. 

Pretty new to python (and obviously SABnzbd code) so let me know if there's anything you'd like me to change.