Adopted typical Ruby conventions for test running.

Restructures the test case files and removes "test-api.rb" in favor of running tests via Rake, with helper and initialization code defined in the "test_helper.rb" file.

It also…

* renames all test case filenames to end with "_test.rb", to aid in filesystem globbing;

* no longer requires you to include new test cases into a parent TestCase instance. Simply create a MiniTest::Unit::TestCase instance for each test case.