Currently there is no efficient way to query for reviews that are

accessible by a given user when using Reviews.objects. This change adds a 

method to ReviewManager such that Reviews.objects.accessible(user) returns a

queryset of reviews that are accessible by the given user.

Created test_review_manager.py file and moved in the ReviewManager

  tests from test_review.py.
Created tests for Review.objects.accessible and ran all tests in

test_review_manager.py with success.