Added ContextAwareAssertions which closes #240

This is a BC. The method is protected so nothing guarantee that some people may extend the AuthorizationService. I may consider this as an acceptable BC though, as this is unlikely to happen. However we must make it clear about how to upgrade in the upgrade guide (https://github.com/ZF-Commons/zfc-rbac/blob/master/UPGRADE.md). Can you update it (under the section "2.3 to 2.4")