[WIP] Moving node validation from JL-Doctrine-DBAL to Jackalope

As suggested in the comments for this code in the doctrine-dbal implementation, this PR will aim to move the node validation to here.

The validate method currently also adds additional node operations (e.g. for node autocreate) to the transport, so I think it should probably be renamed / refactored.
when do we execute these things? during the save operation, right? i wonder if we need some flag to disable (some of?) the auto creation. or move this to a separate protected method so that jackalope-jackrabbit can profit from this class too. things that are handled by jackrabbit (typically generating a really unique uuid) should better be delegated there.

or we could completely ignore this class. i think the only part not handled by jackrabbit is this and thats not too complicated: https://github.com/jackalope/jackalope-jackrabbit/blob/master/src/Jackalope/Transport/Jackrabbit/Client.php#L1291
hm, so maybe lets just move autocreate handling into protected function for added flexibility. @lsmith77 do you have an opinion on this?