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.

* Set the minimum heartbeat to 5 seconds.

* Update unit tests accordingly.
Merge pull request #434 from couchbase/feature/issue_426

Add heartbeat parameter to _changes REST API