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.

```json
{
  "default": {
    "success": true,
    "message": "Application is running"
  },
  "database": {
    "success": true,
    "message": "Schema version: 20140224173010"
  }
}
```

```text
default: PASSED Application is running
database: PASSED Schema version: 20140224173010
```

**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.