Eased merging of two MessageBag instances

This allows you to call `$messageBag->merge($otherMessageBag)` or `$messageBag->merge($validator)`. Useful when working with more than one set of validator instances at the same time, for example.

Currently you have to do `$messageBag->merge($otherMessageBag->getMessages())` or `$messageBag->merge($validator->getMessageBag()->getMessages())`