Converted specs to MiniTest

* Added test_* files matching *_spec files

* Added Rake::TestTask to Rakefile so we can run rake test

* Reworked the md5 digest considerably. Inside the test suite it's now
  just an #md5() method on String and StringIO. Inside the md5()
  method I've removed the clunky, stateful C-style code and replaced
  it with a single-line hexdigest call
Removed RSpec