Rename TestFixtures' public methods

In the module ActiveRecord::TestFixtures:

fixture_path        -> fixture_directory_path
fixture_table_names -> fixture_set_names
fixture_table_names    be reserved for something else, for example 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
Add old methods as deprecated aliases
Use new method names elsewhere
Alias instead of deprecating `set_fixture_class`
Add CHANGELOG entry