Added ability to pass an instance of Graph for GraphHandler methods

Hi,

I stumbled upon this while playing around with the lib... intentionally I expected to be able to work with objects/instances instead of passing graph names to those helper methods.

After merging my changes I saw some more places where I typically like to be able to use an already instantiated object, e.g. https://github.com/triAGENS/ArangoDB-PHP/blob/devel/lib/triagens/ArangoDb/CollectionHandler.php#L261 and following... basically every `*Handler` class contains some parts where the identifying argument is hardcoded to an id or string.

Does anyone agree? If so I'd like to contribute some more stuff on that - just let me know.

Cheers!