Stream heartbeats not honoured

I've been looking for a replacement for avianflu's ntwitter for a while. For all of the many forks, your's appears to be active - and is already on npm, which is an extra bonus. I'll be updating [node-red](http://github.com/node-red/node-red) to use `twitter-ng` shortly.

The issue I found on ntwitter for which this PR fixes is the `checkHeartbeat` function was never being called, so a stalled connected was never detected.

As per [Twitter's docs](https://dev.twitter.com/docs/streaming-apis/connecting#Stalls), if three heartbeat intervals pass with no data arriving, the stream is destroyed.