Modernize Tests

The existing tests were hard to run and didn't work well in Ruby 2.0. This pull request:

* Lists `rake` as a development dependency
* Adds a test task to the Rakefile
* Make the tests load `test/unit` themselves
* Add Ruby 2.0.0 to the compatibility list in the README
* Modernize the `git clone` instructions in the README to use `https` instead of the git wire protocol
* Update the testing steps to simply be `rake`