Rename ActiveRecord::TestFixtures public methods

I would like to know what you think about this.  I propose to rename public methods of  `ActiveRecord::TestFixtures` module as follows:

```
fixture_path        -> fixture_directory_path
fixture_table_names -> fixture_set_names
# fixture_table_names  be reserved to return a hash of the form { "fixture_set/name" => "fixture_table_name", ... }
fixture_class_names -> fixture_model_names
set_fixture_class   -> set_fixture_model
```

This extends PR #4254, where i only tried to give consistent and descriptive names to local variables.

I've posted about this on the mailing list, but there were no comments:
http://groups.google.com/group/rubyonrails-core/browse_thread/thread/22491c3963433854

My motivation was to be consistent in how `directory`, `path`, `table_name` etc. are used.  Also to me `fixture_model` is more flexible and descriptive than `fixture_class`.

Please let me know what you think: if you are happy with existing names, or you have better suggestions, or you like some of the proposed changes but not the other.