Resource dependencies with FooDependency classes

This makes view configuration define explicitely the list of
dependencies on the models, route matching to primary key mappings
and query string parameters.
Use SQLAlchemy scoped_session
Query string parameters as cache predicates

Replace the QueryStringDependency with a QueryStringPredicate class that
can extract elements from a request query string and append them to the
cache key prefix.
Merge pull request #13 from deuxpi/increment-model-class-key

Resource dependencies with FooDependency classes