Ability to rollback AS version updates

This JIRA is based on feedback we received after the Andiamo BOF at JBoss World 2010:

>> A fixpack installer that handles version rollbacks would be fantastic.
>> Of course it needs to remain flexible to work with JBoss installs that
>> have been manually modified.

Note that this may be out of scope for community AS 7; e.g. it may be a JON function. However, the AS 7 design of things like how we lay down content on the filesystem should take this use case into account.