Adds distinct queries option for terms_stats charts

Objectif : Offer the possibility to see distinctive queries in the same graph and regroup them, for the terms_stats.

Example : 
![capture du 2014-06-13 14 58 37](https://cloud.githubusercontent.com/assets/1225408/3270073/dc088d1a-f2fa-11e3-86b0-eb4a5cb3c91a.png)

I just add an option in the terms_stats parameters option menu for that.
You just have to check the option "Distinct queries" to see it in action.

It runs as usual if the option is not checked.

I still listen to your advice, thank you!