Support for Edges

Made Base abstract and added half-concrete DocumentBase and EdgeBase to be able to create both, Documents and Edges, via the gem.

Sorry, but breaks existing code of type "MyDocument < Base". Would need to be "MyDocument < DocumentBase". And edges would be "MyEdge < EdgeBase".

Tested with application of my own. But one have to beware, current ArangoDB (<= 1.4.11) has some issues in displaying created edges (under certain conditions). But the edges are existing.