added debug handler: ElasticSearch\Client::addOnCallHandler

It is possible to register a callback, which gets called every time a request to ElasticSearch is made.

It makes possible to create debug tools, such as this *debug bar* for Nette Fw.
![debug bar example](https://f.cloud.github.com/assets/227416/605166/bc53a294-cd0c-11e2-97ad-f33e38e8579b.png)