Add support for symfony2.2 components

The constructor method signature for
````Symfony\Component\Validator\Validator```` has changed in symfony2.2.
It now requires an instance of
````Symfony\Component\Translation\TranslatorInterface```` which will break
in symfony2.1
Hence we sniff for the existence of the class(ugly, but I don't see
another way unless you maintain separate branches but that seems silly for something that isn't a symfony bundle)
Also update composer.json to allow symfony > 2.1

All tests pass when using symfony components 2.1 and 2.2

Comments welcome