Add Travis configuration.

Rebased and squashed all previous commits to keep Django history clean.

Travis configs are checked out from a separate Git repo.

Warn users not to re-enable email notifications.
Rename django.db.backends sql_table_creation_suffix

It's nothing to do with creating tables (any more?), but rather the
test database.
Split django.db.backends sql_create_model.

Large monolithic functions are very hard to override parts of without
copying and pasting large chunks of code, which is fragile and not DRY.

Split this method into multiple, smaller methods which are easier for
database backends to override and extend. We need this to support creating
UNLOGGED tables in the Postgres backend.
Allow use of UNLOGGED tables in Postgres.

Should increase test performance and hopefully work around issues with
the WAL logs filling up during intensive transaction activity when
running the tests on Travis CI.