Add cancalation of replication with id

Starting from CouchDB version 1.2.0, the original replication object no longer needs to be known. Instead a simple JSON object with the fields "replication_id" (a string) and "cancel" (set to the boolean true value) is enough. (s. http://wiki.apache.org/couchdb/Replication#from_1.2.0_onward)

Add support for this to ReplicationCommand and ReplicationStatus.