Add editorial_reviews which returns a list of editorials.

editorial_review returns the first review.
Add test for single and multiple editorials.
Add check for None and fix possible warning.
Merge branch 'editorials' of github.com:adamlwgriffiths/python-amazon-simple-product-api into adamlwgriffiths-editorials

Conflicts:
	tests.py
Merge branch 'adamlwgriffiths-editorials'