Add basic support for the rgba compound type

This is heavily based on the other custom field types,
to the extent that the common code from these should
really be factored out at some point.
Fix a typo in a comment
Add the color column back to Textlabel (finally)
Tidy up the imports from .fields
Add the initial South migration

This migration should create a database with a scheme very similar
to one generated with the PHP migration 2013-02-01T13:53:00

The key differences are:

 - DEFAULTs will not be in the schema for most tables.
   South doesn't create these, on the basis that for a project
   where the database is only being accessed via Django's
   ORM, the default keyword argument is sufficient.

 - The types of a couple of columns have changed from bigint
   to integer in cases where we couldn't possibly need more
   than a 32-bit signed value.

 - Sequences are created slightly differently, but not in
   way that's functionally different for us.

 - COMMENT ON TABLE comments are no longer created.

 - We don't create a UNIQUE constraint on primary keys, since
   PRIMARY KEY already implies UNIQUE.

These have been checked by diffing a current database dump
and one from a database created with the South migration, which
is tricky to do precisely, but gives me some level of confidence
that it's created an equivalent database for the purposes of
CATMAID's Django backend, at least.
No longer call the PHP migrations