Add a window for heartbeat pings and pongs

[inhies]

I was having issues with my test system cosing the connection due to imprecise timing for the heartbeats. If the client using stomp.js says it will send a heartbeat every 10 seconds, the server killed the connection as soon as 10 seconds has elapsed. This was a problem because stomp.js was setting a timer for 10 seconds exactly but it wasn't sending the heartbeat in time.

The heartbeatWindow value allows you to specify a percentage difference. By default I set it to 10 percent, which will send a heartbeat every 9 seconds and expect a heartbeat every 11 seconds, if using the default 10 second values for the heartbeat negotiation.

[jmesnil]

it's not necessary to modify the timeout value for the heart-beat received from the broker, the library is tolerant and will not report a connection issue before 2 missing heart-beats (see https://github.com/jmesnil/stomp-websocket/blob/master/src/stomp.coffee#L143)

If you modify your commit to only apply the heartbeatWindow to the heart-beat sent by the client, I will merge it.