Implementation for prune revision tree (equivalent of _revs_limit), except for modifying compact() method (#145)

https://github.com/couchbase/couchbase-lite-android/issues/145
Call prune revision tree in compact() method (#145)

https://github.com/couchbase/couchbase-lite-android/issues/145
Update to pull request #76 from b-smets -- looks like it cause another unit test to fail.  This change fixes that failure.