Add support for coloring the test output

I have added support for colorizing the output emitted by the `Xcode:Test:Formatters:StdoutFormatter` class. It basically just emits a red/green dot and summary text to given you a quick indication of pass/failure.

I added a dependency on the Colorize gem, but this could easily be dropped in favor of internal control codes if its a big deal.

It's defaulted to enabled at the moment, but I partially wired support for configuration -- we may want to design a pattern for passing options in from the `test` method into the formatters (i.e. `:stdout_formatter_options => { :colorize_output => true }`) or I could add a class level configuration for it.

Let me know if there's interest in merging the support and what you are thinking in terms of changes.