Support evolving unique_together.
Django-Evolution has always recorded unique_together, but never

supported evolving it. If a project changed this value, it would be

ignored and never applied to the database, yet would be recorded.
This introduces a new ChangeMeta mutation that accepts changing a Meta

property. Currently, only unique_together is supported. The

unique_together property can be set to any value, and Django Evolution

will attempt to drop any old indexes it can find (if any exist) from the

old signature, and set up indexes for the new signature.
Projects should add a ChangeMeta for any models that have a

unique_together, to ensure that the proper value is used and recorded.

Unit tests pass for SQLite, MySQL, and Postgres, with Django 1.4 and 1.5.


Installed a Review Board 1.6.x site with Django Evolution 0.6.9 using MySQL.

  Then, with RB 2.0 and this patch, I checked evolve --hint. It listed evolutions

  for all the unique_together fields.


Added those evolution files and ran evolve --execute. Saw that they applied.


Also tested all this with my old database that I've carried over since 1.0 (and

  had newly stored unique_together fields but no actual indexes for them). No

  problems.