Add setup_interface_logging() to execution docs.

Correct the programmatic execution example so that it, like run(),
configures interface logging.

Fixes the runtime error that occurs when following
the current docs/programmatic_execution.rst, namely:

    No handlers could be found for logger "luigi-interface"
Merge pull request #581 from hblanks/interface-logging-in-docs

Add setup_interface_logging() to execution docs.