1. Fix code to pass failed tests:
   - Issue6_ModelShouldBePassedToLayout;
   - Issue21_SubclassModelShouldBeSupportedInLayout
2. Make Include() model parameter optional. Pass current page model if it is specified and null is passed to the Include() method. Added tests for these cases.
Merge pull request #74 from AlexBar/release-3.0

Model is not passed to Layout