Setup and apply new Scalariform rules

This PR applies the following Scalariform rules to Gatling's code :

* Indent with 2 spaces 
* Double indentation of class declaration : https://github.com/mdr/scalariform#doubleindentclassdeclaration
* Align multiline class and method parameters : https://github.com/mdr/scalariform#alignparameters
* Align single line case statements when case under < 40 chars (default) : https://github.com/mdr/scalariform#alignsinglelinecasestatements
* Indent local functions : https://github.com/mdr/scalariform#indentlocaldefs

As usual, those rules are automatically applied before compilation by SBT.