Add new Entity class to simplify holding attributes and remove attribute namespace

[clue]

// old
assert($graph instanceof \Graphp\Graph\Attribute\AttributeAware);
assert(is_array($graph->getAttributesBag()->getAttributes()));

// new (applies to Graph, Vertex and Edge)
assert($graph instanceof \Graphp\Graph\Entity);
assert(is_array($graph->getAttributes()));

Builds on top of #182, #185 and 186