Check for invalid characters in Documents

I've tried to implement check, that should throw a PHPCR\ValueFormatException, when some dangerous <control> sign occurs in document, that should be saved in a repository.

This as alternative version, how to implement this PR: https://github.com/jackalope/jackalope-doctrine-dbal/pull/99

I hope I haven't messed it up too much, this is my first PR ever..