deduplicate albums() and items() methods

This also drops the keyword arguments to these methods, which were vestigial.
simplify get_query using class attributes
consolidate excess SELECT in load()
fix outdated _record reference in web plugin
Added opus support, maybe.
Opus now passes basic tests.
finish up changelog for beets 1.3.0
Added tag v1.3.0 for changeset 87945a0e2175
Allow the convert plugin to convert to any format, not just mp3.
Removed reference to my testing plugin, oops.
Problem with unicode filenames sorted.
version bump: 1.3.1
Merge pull request #381 from rowan-lewis/opus

Added opus support
opus: sample rate is always 48kHz
opus: remove image from test file

The encoder that produced this file for some reason included an empty image as
the cover art and was confusing the tests.

I left the null check in place to deal with this situation in the future. I
think returning None is better than returning the empty string (which is of
course not a valid image).
opus: changelog/thanks/docs (closes #377, #381)
embedart: fix error message when imagehdr fails
docs: mention musicbrainz port config (closes #365)
Changed the configuration to allow for easier format selection.
It's called Vorbis, not OGG.