Since seq replication

I tried to use only String but that doesn't work for standard CouchDB. CouchDB expects a long (or to be more precise a `since=` without quotes around the value. (it does work for Cloudant though). I used Object as type of the since_seq and dynamically detects what type is is (is it parseable as Long) and build the `ReplicationCommand` with the proper type so that it's serialized by Jackson correctly. Updated the tests as well.