Make Kdyby/Translation optional

[enumag]

Currently Kdyby/Validator requires Kdyby/Translation which doesn't work at all without HttpExtension and SessionExtension. Since there is no reason why Validator should require SessionExtension we should break the chain somewhere.

In my opinion we should remove the dependency on Kdyby/Translation from this package. Someone might want to replace it with some lighter integration of symfony/translation (it's on my todo list).

@fprochazka What do you think?