Code design

* composer.json file:
    * Namespace for tests moved in a dedicated section
* README.md file:
    * HHVM badge added
* src/* and tests/* files:
    * Code design reviewed to be compliant with PSR-1 and PSR-2
I think we can remove this and use `\LogicException` - having an exception in this class is also a PSR0/4 violation.