Tests for `Color` class

I set up MiniTest as discussed in #11. Tests can be run using `rake test`. I added tests for the `Color` class.

Running the tests surfaced an error in `LearnRubyError` (a dependency of `Color`). `LearnRubyError` would color its output by calling `String#error`. However, the `error` method was monkey patched onto `R2D::String` and not `String`. Moving the monkey patch outside of the namespace fixed the issue.

Go ahead and criticize/comment. Once the tests are to your liking, I'll do the other classes.