Added TimestampedMigrationAttribute

Great job with fluentmigrator.  We love it around here!

I added a class, TimestampedMigrationAttribute, which extends MigrationAttibute.  This class derives the underlying "Version" value from the given year, month, day of month, hour, minute, and (optionally) second.  It also provides constructors that accept TransactionBehaviour and Description, so it provides the same instantiation options as the base type.

We're using something similar on a huge project to limit the amount of effort we put into "claming" the next ordered migration.  We just create a TimestampedMigration instead, and only worry about conflicts when migrations created by different people actually conflict with one another.