Remove unused imports.
Make Config no longer dependent on filesystem objects. Ref bug 959728.

This saves us a file write during SingleFileTestCases.

build_instance will likely change to take a dict-like object later, making it even more manipulable, once we've converted to configobj.
Fix config bug wherein specifying `*` for `enabled_plugins` or `disabled_plugins` wouldn't take all the plugins into account.

Also remove the last vestiges of a single plugin folder. You'll need to remove references to plugin_folder in your config file if you had them and instead register your third-party plugins using setuptools entrypoints, as described at http://dxr.readthedocs.org/en/es/development.html#registration.