Add rake task for changing timestamps to UTC

Rails 2 used local times, Rails 3 assumes by default times are stored in UTC.
This rake task changes the timestamps in the database to UTC.
Merge remote-tracking branch 'opf/feature/rails3' into fix/change_timestamps_to_utc

Conflicts:
	doc/CHANGELOG.md
Merge pull request #578 from opf/fix/change_timestamps_to_utc

Add rake task for changing timestamps to UTC