Include pass/fail information in text and JSON output

As you add more OK Computer checks, it can sometimes be hard to pick out the one that's currently failing. This change adds "PASSED" or "FAILED" to the text output and adds a "success" boolean to the JSON output so that failure can be programmatically detected.

**Note**: This is a breaking change to anyone depending on the current JSON format, so would necessitate a version bump. This is the wild-west of pre-1.0, so I'm not terribly worried about this.