Improved Configuration::load
Improved Configuration::load

* Changed RuntimeException to more specific InvalidArgumentException
* Improved exception messages
* Extracted Configuration::load into
Configuration::importExtendedConfiguration and
Configuration::processEventSubscribers
* Moved Configuration::importExtendedConfiguration call before
Configuration::validateSchema

accompli/accompli,accompli/accompli,niels-nijens/accompli,niels-nijens/accompli