Require ChangeMetas for existing models containing unique_together.
Django Evolution previously stored but otherwise ignored unique_together

entries, meaning that older values could not be trusted to have been

applied to the database It could be wildly inconsistent with what's

actually there.
This change considers an older unique_together to be invalidated,

requiring a ChangeMeta mutation, if from Django Evolution 0.6.x or

older.
There's a new flag stored in the signature for each table called

__unique_together_applied. This is used to indicate whether the

field has been applied to the database (or, in the case of an empty

unique_together, at least considered).
When diffing, two identical unique_together values are considered

to be unequal if the older signature doesn't have this new flag.
Projects with existing unique_together values will need to provide

a ChangeMeta for each table. Samples are shown when using evolve --hint.
This also provides some built-in evolutions for the main affected Django

models.

Unit tests pass for SQLite3, MySQL and PostgreSQL for Django 1.3, 1.4 and 1.5.
Tested against Review Board without any ChangeMeta evolutions. It told

me I needed an evolution for every unique_together we had. Ran evolve --hint

and saw the suggestions for each.
I then added evolutions for all of these and ran evolve. It said the evolutions

looked good and would apply cleanly.
Ran evolve --execute and it applied all my evolutions. Checked in the database

to be sure they were there.
None of the existing indexes were removed or replaced.
Created a brand new model with a unique_together and ran syncdb, followed by

evolve and evolve --hint. It didn't complain about missing evolutions. Verified

the new unique_together was in the database (that part is Django's job) and

that the correct flag was in the signature.
Made a change to that unique_together and saw that it properly told me I needed

a mutation, which applied successfully when doing evolve --hint --execute.