Add stbt._set_config

`stbt._set_config` allows writing stbt config files.  This is required for Smart TV support to allow storing the results of calibration, but it may be useful elsewhere.

`stbt._set_config` is private and not exposed via the `stbt config` command as, due to limitations of Python's `ConfigParse` it will currently remove all comments and normalise all whitespace in the configuration file.  We may be able to [fix this in the future][1] such that we can provide the function as part of the public API.

[1]: http://stackoverflow.com/questions/6620637/python-configparser-question-about-writing-comments-to-files

These commit have been split out of the much larger Pull Request #100 - Smart TV Support to make it easier to review and integrate.