Code design

* composer.json file:
    * Namespace for tests moved in a dedicated section
* README.md file:
    * HHVM badge added
* src/NumberTheory.php and test:
    * RootSquareException replaced by LogicException
* tests/unit/MathTest.php:
    * ` and ` replaced by ` && `
* src/* and tests/* files:
    * Code design reviewed to be compliant with PSR-1 and PSR-2