Fixed invalid JSON formatting and broken unit test

Fixed invalid formatting in JSON output:

- `TimeUnit` turned into `string` as not a valid data type itself.
- Added missing comma between metric values.
- Turned percentiles into an object property with key for each percentile.

![image](https://cloud.githubusercontent.com/assets/131293/3960348/1dcf0ef0-273e-11e4-909a-962ab72d798d.png)

Fixed the `MachineMetricsTests.Can_load_all_metrics unit` test:

- Passed the Metrics instance into the MachineMetrics constructor so that the performance counter metrics can be added to the correct collection.