Add View Variables plugin
Handle pre/post #261
Remove unneeded import of track.
Show variables for tracks without files
PICARD-575: load plugins from contrib/ too when run from source directory

http://tickets.musicbrainz.org/browse/PICARD-575
Show multi-value variables as lists.
Fix display of multi-value for id3v23
Add Album Artist Website plugin - see PICARD-26

Adds a plugin to get the artist homepage MB relationship and insert into
website (ID3 WOAR tag).

Note: I used this plugin as a trial for using a Picard xml module to
handle MB WS2 xml response. See PICARD-581.
Add NEWS.txt
Remove unneeded imports
Minor tweak to display if variable is null string
Handle album finalisation correctly.
Move ID3Metadata functionality to new_metadata property

The problem with ID3Metadata is that it changes the behavior of
Metadata methods in undesirable ways (see PR #273).

This adds a new_metadata property to File classes which returns a copy
with the values modified as they'd be displayed/saved. This should more
composable and less error-prone than overriding Metadata methods across
the board.
Add `update_constants` command to setup.py

It retrieves and parses attributes.pot and countries.pot files
from transifex to extract various constants needed in Picard.
It helps to keep those in sync with MusicBrainz website.

It then generates:
- picard/attributes.py containing cover art types, medium formats, and release group types
- picard/countries.py containing countries

`update_constants` command replaces `update_countries`
Update picard/attributes.py and picard/countries.py

New setup.py update_constants command was used.
Add ugettext_attr() function needed to translate attributes

Since contexts are used in attributes.pot, and python gettext do not yet
support contexts, a minor hack is used to make it work, this workaround
is used in django and babel, see ugettext_ctxt().
Build RELEASE_FORMATS and CAA_TYPES from MB_ATTRIBUTES
Ensure proper translation of cover art types
Build RELEASE_PRIMARY_GROUPS and RELEASE_SECONDARY_GROUPS from MB_ATTRIBUTES
Build release type score sliders UI dynamically.