Add a model => class mapping capability

For those people who don't like to have all their models in the global
namespace (which is anyone who's ever tried to model the relationships
between books using a table called "sequels"...), fixture_dependencies
hasn't been much use, because it can't find the class.

This patch adds a new `FixtureDependencies.model_class` hash, which can be
filled in with `:model_name => Model::Class` mappings, so the fixture loader
can find the class it needs.  Backwards-compatibility is maintained by using
the existing algorithm (constantize, etc) if no entry matching the model
name is found in `FixtureDependencies.model_class`.