Add heartbeat parameter to _changes REST API

Add heartbeat (CRLF ping) to _changes REST API in continuous and longpoll mode. The heartbeat parameter will accept a value in millisecond and the value needs to be more than zero. #426
I think there should be a minimum value, for sanity's sake. Maybe 5 sec? We don't want the router to start sending CRLFs every millisecond! This would be an easy mistake to make if you forget that the time units are milliseconds, and use `?heartbeat=30` ...