Autogenerate doesn't correctly handle postgresql enums

Migrated issue, originally created by Ulrich Petri (@ulope)

According to #67 enum types used in columns have to be explicitly created and destroyed.

However the autogenerator currently doesn't produce those explicit commands. This problem is made worse by the fact that the autogenerated migration appears to work fine the first time it is applied and even removed. The dangling enum type only causes a conflict the second time the migration is applied.

edit by mike: note also that anything we do for PG native enums also has to run outside of a transaction block, so this is really not going to be easy. #123 provides the feature for this however if autogenerate, or the operation itself, is going to make that happen automatically, that's a very dramatic behavior.

Start by tracing the autogenerator’s handling of PostgreSQL enum types, then read issue #67 for the required explicit create/drop behavior and #123 for transaction handling. Done means regenerated migrations manage enum types explicitly and account for the requirement that native-enum operations run outside a transaction block.