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 we should set a minimum. SG uses 25 sec; not sure if CouchDB uses the same. Since this is only used to keep idle sockets from being closed, there's no need to send it very often.