Improved heartbeat controller to engine monitoring for long running tasks

I'm using Ipython parallel for long running tasks (hours to days) on larger clusters (100+ engines) and have been having trouble losing connectivity with engines. The ipcontroller log would intermittently report a block of 5 to 10 `registration::unregister_engine` messages and remove those engines.

In digging into the problem, it appears as if the current behavior is to ping engines every 3 seconds and kill an engine if it fails to respond to pings twice. The first failed ping results in an engine being put `on_probation` and the second triggers unregistration.

For longer running engines this is too restrictive, and is at odds with the new `EngineFactory.max_heartbeat_misses` which defaults to 50 misses.

This pull request provides a configuration variable `HeartMonitor.max_heartmonitor_misses`, which allows ipcontrollers to specify how many consecutive misses to allow before unregistering an engine. It defaults to failing to contact an engine for 1 minute.

I'm happy to adjust or include different defaults if desired. Thanks much for all your work on IPython.
Brad