Added collection modification requesting ability

[dinoboy197]

I've added the ability to request modifications to collections based on what XEP-136 has to say about replication (http://xmpp.org/extensions/xep-0136.html#replication). This should allow for a client to replicate the archive locally, by requesting modified collections since a given date and then retrieving those individual collections and messages which have been modified using the existing request handler.

Specifically,

1. Added a version number for each archive collection, which starts at 0 and is incremented any time that archive collection is modified (adding a new message, for instance)
2. Updated the database version from 2 to 3 since I'm adding a version number
3. Added the ability to the JdbcPersistenceManager to query for conversations modified since a given date
4. Added an XMPP IQ handler to process and return results for queries

I also added a couple of GPLv2 notices to files that I worked on a substantial amount; hope this is ok.

I'm happy to discuss any implementation details.

[dinoboy197]

Any comments on this?