Handle Performer tags stored in ID3v2.3

Performer tags are stored as part of the IPLS tag in ID3v2.3 and this is
put into the TIPL tag when read by mutagen. This fix adds unknown TIPL
roles as Performer tags.
Remove sorting for displayed multi-value tags and provide a sort tags plugin

Picard stores tags unsorted but displays them sorted and this is
inconsistent.

This commit removes display sorting of multi-value tags from Picard
improves consistency by displaying what will be / is actually saved.

This commit also provides a plug-in to allow user to sort tags both
displayed and stored in the file.
Add option for user to set concatenation character(s) for id3v23 multi-values
Improve displayed values and comparison  when saving id3v23
Fix original values metadata class after save
Handle TypeError on flattening
Address peer review comments

Improved object handling on file-save. Improved code for identification
of id3v23 true multi-value fields. etc.
Remove spurious exception handler.
Resequence change notes.
Fix terminology.
Another minor terminology tweak in News.txt
Remove incorrect comment.
Fix function and white space
Modify and move setup_gettext() in its own file.

Unit tests will need to use gettext, so i modified a bit setup_gettext()
and it can now be easily imported.
I add to change the code a bit to make it work, i also fixed an issue with
locale under linux that was occuring when one imported this module.
Add functions to convert bytes to human readable form.

Binary and decimal modes are supported (MB and MiB ie.)
It supports i18n using gettext and locale.
Precision can be modified if needed, by default it is using 1 digit (if needed).

Extensive tests were written, the toughest was to make them work for
both default C locale and fr_FR.UTF-8 locale (ofc it is possible to test for more
locales...). If one locale isn't available on testing system, test is skipped.
fr locale was chosen because decimal point is replaced by a comma and byte units
becomes "octet" units (1.5 MB in english -> 1,5 Mo in french).
Use bytes2human to display file size in Info dialog.
Display cover art images sizes and dimensions in Info dialog
Handle id3v23 date / original date in yyyy only

For id3 files where picard is set to write id3v23 tags, truncate dates
provided by MB to yyyy (from yyyy-mm-dd).
Fix white-space issues as pep8
Remove some obsolete code

Python 2.5 is no longer supported.