Call shutdown() method on the ProcessContainer when shutting down sensor
manager using CTRL-C.

This fixes a bug with stray processes.
Also make sure we call sensor_container.shutdown() on SIGTERM.
Update changelog.
Use .keys() instead.
Fix some typos.
Merge branch 'master' into fix_sensor_container_subprocess_shutdown_bug

Conflicts:
	CHANGELOG.rst
Merge pull request #930 from StackStorm/fix_sensor_container_subprocess_shutdown_bug

Fix a bug with child processes not being killed when stopping a sensor container