Timestamp migrations

This commit adds timestamp based migrations to Doctrine 1.2.4.  In the biblesearch repository there's a timestamp-migrations branch that implements this code.  To convert from version number to timestamp-based migrations, run ```./doctrine upgrade-timestamp-migrations```.  This ought to update the database to the proper state based on the version number currently in the table and the current list of files.  If those are out of sync, then the upgrade may need some tweaking by adding or removing rows from the ```migration_version``` table.

When a version is specified for ```./doctrine migrate``` it'll migrate up or down all the versions between the newest timestamp and the specified timestamp.  I also added ```./doctrine migrate-up``` and ```./doctrine migrate-down``` to run the up or down method only on the migration specified.

The original work for timestamp based migrations was completed by David Rowe over here: http://www.doctrine-project.org/jira/browse/DC-471