path_sep_replace config option

I also took this opportunity to move and rename util.santize_for_path to
library.format_for_path, which was long overdue.
configurable max filename length
look up max filename length automatically
docs/changelog: filename length detection (GC-461)
original_{year,month,day,date} fields (GC-441)
original_date config option (GC-441)
Clean up imports per PEP8
Remove obsolete globals
Clean up and extend comments and logging statements
Back to using echonest_ prefix for db fields after reading mailing list thread on the problem
Import beet.config as config
Remove irrelevant syspath call
Minor cosmetic changes
Make grabbing audio_summary information not optional
Don't filter out album candidates, but yield them in frequency order

Since Echonest tends to push out a lot more tracks per song (a concept
that MB doesn't appear to have), it is a lot more likely that using
the proportion of times a release/album appears in a group of
songs (here tracks) is going down.
Document template_field function generator function
Merge branch 'master' of https://github.com/sampsyo/beets into echonest_plugin
Initial commit for echonest plugin rst documentation