python packagisation: Move stbt._named_temporary_directory to stbt.utils
python packagisation: stbt.ConfigurationError is Exception not UITestError

This is to avoid a circular import when I move ConfigurationError to the
"stbt.config" sub-package. This is a breaking change.

ConfigurationError inherits from Exception rather than another more
specific exception, so that we have an upgrade path if in future we
decide to change `get_config` to raise `TypeError` and/or `ValueError`.
python packagisation: Move stbt.get_config etc to stbt.config sub-package
python packagisation: Move logging infrastructure into stbt.logging

Mostly copied from a patch by Will Manley:
https://github.com/drothlis/stb-tester/pull/201