Convert the README to md syntax and add tracking code for githalytics.com

See http://githalytics.com/

The snippet is
```
[![githalytics.com alpha](https://cruel-carlota.pagodabox.com/ed9166f60bd7a26c93f0bb34dca7f69b "githalytics.com")](http://githalytics.com/gem/oq-engine)
```