Graph Traversal

[tjoubert]

1. I've implemented graph traversal in this PR. Although the functionality has been implemented in the GraphApiClient, it does not fit our traditional pattern of one client method per REST endpoint. One method takes an object, convert it to an AQL query for graph traversal and executes it through the second method which takes an AQL query and executes it in the database through the CursorApiClient.

2. ApiClientBase now exposes a new member protected ArangoDBClient _arangoDBClient; and accepts a new parameter in its constructor ArangoDBClient arangoDBClient = null. The parameter is null by default to ensure backward compatibility. The above features mean that the client is now aware of the ArangoDBClient it is associated with and has the ability to use its other features. This was necessary so that we could consume the CursorApiClient functionality from within the GraphApiClient to do the traversal.