Allow addEdge() to take vertex ids in the JS library

**Original Issue: 528**

Currently, this works:
```javascript
v1 = graph.addVertex();
v2 = graph.addVertex();

z = graph.addEdge(v1, v2, null, {"name":"john smith"});
```

However, sometimes, I have the ID of a vertex, but not a reference to
it. I can of course, ask ArangoDB to retrieve the vertex and then use it
with `addEdge()`, but that's not efficient.

I would love to be able to do this:

```javascript
z = graph.addEdge('myvertexcollection/123', 'myvertexcollection/456',
null, {"name":"john smith"});
```