New algorithm, "alpha"

Also refactored constant.py to be a usable base class for bitrate
protocols.  Since "constant" is now large, "minimal" is a
simple-minded algorithm that just sends at 1Mbps.
Write progress bar to stderr, so that you can redirect "harness" output.
Set better grath limits and smooth probability more for draw_trace
Graph fixes for draw_trace

 + Scatter points to provide better estimate of density
 + Use opacity on points for better estimate of density
 + Order the bitrates by throughput, not rix
Split the "out" folder by algorithm.