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`.
Specs for model_class map
Specify class relationships on both sides
Minor changes to class name overriding support

Rename model_class to class_map, and model_klass to model_class.

Add documentation to the README about this, and bump version to
1.4.0.