Emit milestone event using core modules.

Emit ``milestone`` event using core modules (remove dependency on ``windowPerformance`` module).

Example:

```
phantomas/examples$ ./promise.js 
(...)
Loading progress: 10%
Loading progress: 30%
Milestone reached: responseEnd
Milestone reached: domInteractive
Milestone reached: domReady
(...)
Loading progress: 100%
Milestone reached: domComplete
```

Updates #240