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
Heartbeat isn't on by default. The [CouchDB docs](http://docs.couchdb.org/en/latest/api/database/changes.html) do talk about a default value, but I think that means the value you get if you specify `heartbeat=true` ("Default is 60000. May be true to use default value.")

This is important because we don't need or want a heartbeat for a local (PhoneGap) app. It'll just waste CPU.