Add helper_run setuptools extension.

This PR installs a new setuptools command named run_helper which runs a `helper.Controller` sub-class in the same manner as `helper.start`.  It is installed as a `distutils.command` entry point as described in [Creating distutils extensions].  This makes it available as a command in any *setup.py* where *helper* is in the environment/path/whatever.

The command itself takes two parameters:
* --configuration=FILE / -c FILE
* --controller=NAME / -C NAME

The latter is identical to the `-c` option exposed by `helper`.  The `-C` option controls which controller is imported and run.  I considered using `importlib` instead of `__import__` but I couldn't convince myself that another back port dependency was justified.

Note that I did not bump versions or update the mini changelog.

[Creating distutils extensions]: http://peak.telecommunity.com/DevCenter/setuptools#creating-distutils-extensions