web: Better HTTP server

[GoogleCodeExporter]

To serve the "web" plugin's web interface, we should transition to a better 
server than Flask's built-in development server (which is kind of slow and 
raises spurious "broken pipe" errors). One good candidate might be Waitress:
http://pypi.python.org/pypi/waitress

Waitress support high-performance file-sending (via sendfile), which is great 
for streaming audio.

Original issue reported on code.google.com by adrian.sampson on 3 Apr 2012 at 10:25

[untitaker]

Do not comment here, but on sampsyo/beets#195