Add baselines for unique_together for Django Evolution 0.7.
unique_together changed were ignored by Django Evolution in the past,

but that's no longer the case with 0.7. However, projects do need to

establish a baseline for unique_together now, to get the database back

in sync with the evolution signatures.
This adds a ChangeMeta for every unique_together we use, in order to

establish that baseline.

Successfully evolved my database with Django Evolution 0.7, and saw

that the constraints in the database were set up properly.
Tested with MySQL and SQLite.