Add filter and average function to DB collection
after lesson 8
Merge remote-tracking branch 'remotes/oggetto/master'

Conflicts:
	src/models/ProductReview.php
	src/models/ProductReviewCollection.php
	src/models/Resource/DBCollection.php
	src/models/Resource/IResourceCollection.php
	tests/ProductReviewCollectionTest.php
	tests/ProductReviewTest.php
fix merge