Add support for retrieving the set of active tasks.

There's some slight nastiness here in StdCouchDbInstance.getActiveTasks. There's an ActiveTask interface and a StdActiveTask abstract class. The return type of getActiveTasks is Collection\<ActiveTask\> but within the method we build a List\<StdActiveTask\>, so we have to copy that list to a List\<ActiveTask\> before we can return it. We have to use StdActiveTask rather than ActiveTask to do deserialization properly.

Other possible ways to go about this:

1. Make the return type Collection\<? extends ActiveTask\> instead. If we do this, we can no longer write Collection<ActiveTask> foo = dbInstance.getActiveTasks();

2. Get ride of the ActiveTask interface and just have StdActiveTask