'current_timestamp' field type to support MySQL 'current_timestamp' magic.

There was no way to set the `CURRENT_TIMESTAMP` flag on a `TIMESTAMP` field with the existing options, and I didn't want to have to do it in my code somewhere.

It will set the timestamp field to be equal to the current timestamp upon register, and then when the record is changed, it will be automatically updated to the current timestamp. Basically for an `updated` field :-)

*Sorry about the merge commit in there... I can fix it up and provide a clean one-commit version if you'd prefer?*