Add support for symfony2.2 components

The constructor method signature for
Symfony\Component\Validator\Validator has changed.
It now requires an instance of
Symfony\Component\Translation\TranslatorInterface which doesn't exist
in symfony2.1
Hence we sniff for the existence of the class(ugly, but I don't see
another way)
Also update composer.json to allow symfony > 2.1
Merge pull request #37 from tommygnr/symfony2.2-support

Add support for symfony2.2 components