XcodeColors support

**Do not merge this yet. I'm issuing this pull request as an invitation for feedback.**

PR for Issue #357

---

### `KWTestObserver`
This class registers itself to replace the default `Sen/XC``TestObserver` so it can print color codes before and after test notifications are printed to the console.

It is only intended to be used when the Xcode plugin [XcodeColors](https://github.com/robbiehanson/XcodeColors) is installed, and only registers itself if the environment variable `XcodeColors` is set to `YES` *(The method described by XcodeColors)*.