benchmarking for slow matches

Investigating #609. The culprit seems to be repeated reading of the config's
weight settings.
style fixes in match.py
avoid unnecessarily reloading weights (#609)

In my profiling runs, this was taking 80% of the matching time to recompute
exactly the same dict over and over. Worth optimizing.

`beet bench_match` went from 80 seconds to 17 seconds on my machine with this
change.
benchmark: customizable album ID
Merge branch 'master' of github.com:sampsyo/beets