Add namespace support to DispenseableResource

Which means, that you can use:

$resource = new \Enable\Store\DispenseableResource($graph);
$resource->init();
$resource->addNamespace("enable", "http://...");
$resource->foo = "bar";
$resource->{"enable:ttt"} = "namespaced property";

...

$resource->load();

// if namespace "enable" was registered before, it will
// resolved and you can use it again instead of the whole
// URI
echo $resource->{"enable:ttt"};