Extensible lock service

This pull request contains work based on the forum thread http://forum.liquibase.org/topic/extending-liquibase-to-support-cassandra#49382000000604053. There are a few changes. First, LockService has been refactored such it is now an interface and the original class has been renamed to LockServiceImpl. Secondly, instances of LockService are now obtained using the new LockServiceFactory. Lastly, ServiceLocator has been updated to scan the package liquibase.lockservice.