Various fixes and improvements

* Make ConfigSlurper respect Environment on reload
* correctly slurp external config files (though ignoring classpath resources for now, should be easy to add)
* make service method easier callable from services (map instead of parameterMap)
* allow to overwrite a setting instead of having to delete and recreate
* allow to add a config value which didn't already exist (could be enabled, disabled via option or programmatically)