Adopted typical Ruby conventions for test running.

Restructures the test case files and removes `test-rails.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;

* moves test case files into either a `rails` or `application` directory, based on the scope of the test;

* no longer requires you to include new test cases into a parent `MiniTest::Unit::TestCase` instance. Simply create `MiniTest::Unit::TestCase` instance for each test case.