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 2.5 seconds is much too short; it's going to result in too much network traffic (or CPU if it's local). CouchDB uses 60 sec; if that seems to long then maybe 30 or 25.

Also, I'd like to keep time intervals specified as seconds since that's what Cocoa uses (NSTimeInterval). You can do the conversion to/from milliseconds when reading/writing the value.