config: 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