Add %_live%, %_cover% etc. for work relationships.
Add NEWS
Switch variable names and add ~workrelationship

which mimics the work relationship statement on the MB web site e.g.
`{live} {medley:medley including a} {partial} {instrumental} {cover}
recording of`
Handle i18n differently ...

... so that work relationship text can be translated as a phrase and not
as individual words.
Fix metadatabox sort order

Sort by displayed value not internal value.
Merge pull request #297 from Sophist-UK/sophist_fix_metadata_display_sort_order

Fix metadatabox sort order
Remove ~workrelationship stuff.
Merge pull request #285 from Sophist-UK/sophist_add_work_relationship_variables

Add %_performance_live%, %_performance_cover% etc. for work relationships.
Fix sort multivalue tags plugin

Exclude certain multi-value tags from sorting because the order is
important.

Get raw list because id3metadata sub-class overrides the getall method.

Make a class to avoid polluting the global namespace with the
exclude_tags variable.
Expand list of excluded tags

and move back to non-class form.
Very minor style tweak
Explain exceptions and fix typo.
Simplify slightly.