merge patch for timestamp based migrations

Original patch: http://www.doctrine-project.org/jira/browse/DC-471

modifications to merge cleanly with 1.2.4 and use timestamp based class names
add task to upgrade migrations table
add unique index to timestamp_value
add migrate-up and migrate-down methods
add pending migrations task

remove special handling of -1 timestamp to list executed migrations
Merge pull request #1 from agathongroup/timestamp-migrations

Timestamp migrations