SCOP improvements

**Note: Do not merge until the server has been updated.** This can be verified by getting valid XML (not the DOMAIN_SERVICE_SERVER message) at http://source.rcsb.org/jfatcatserver/domains/getComments?sunid=127355

This merges a number of improvements relating to SCOP:
* Fix #52: Support older SCOP versions in RemoteScopInstallation
* Fix #50: Error in RemoteScopInstallation
* ScopFactory behave more consistently
* Support Comments in RemoteScopInstallation
* Much more comprehensive unit tests for all ScopDatabase implementations using parametric unit tests
* Better error handling

Some of these features required changes to the server at source.rcsb.org, which will be deployed shortly to production.