Add YAML configuration file support

[kpinc]

The sample YAML config files do not have %(here)s, or any other
interpolation.  (The plaster-yaml interpolation code is broken,
and, in any case, should probably be removed and replaced with
template substitution.)

The %(here)s are removed from testing.ini, in favor of a relative path.

This is the version that does not need the plaster.Loader.get_settings() 
"raw" parameter patch.