mpdupdate: Allow UNIX domain socket for MPD server

If the host configuration begins with a '/', it is assumed to reference
a UNIX domain socket. This is similar to libmpdclient's behaviour.
Merge pull request #313 from jhawthorn/mpdupdate_unix

mpdupdate: Allow UNIX domain socket for MPD server
mpdupdate domain sockets (#313): changelog/docs
preferred_media: null default, expand docs

Setting the default preferred_media to null is more like previous versions.
This way, as digital becomes more popular, we aren't stuck with a default
configuration that prefers an outdated format.
media field distance: binary, not string diff
media comparison: lower-case both sides
Added tag v1.2.0 for changeset b3f7b5267a2f
oops! forgot release date
Added tag v1.2.0 for changeset ecff182221ec
Merge branch 'master' of github.com:sampsyo/beets into distance-refactor

Conflicts:
	beets/autotag/match.py
	beets/config_default.yaml
	docs/changelog.rst
	docs/reference/config.rst