reader: fix heartbeat_interval bug

* conn not self for heartbeat_interval
* chmod +x and add shebang to example
* consolidate heartbeat handling into Client
* improve test flakiness and duration by intelligently waiting for nsqd
* add test for reader heartbeats
Merge pull request #69 from mreiferson/heartbeat_interval_bug_69

heartbeat_interval bug when checking last received timestamps (sigh)