Decide/document how to update a vertex

[buchanae]

Currently, updating an existing vertex requires first calling Get, then calling Add.

That's perfectly fine; for example, Google Datastore has this behavior. It should be at least documented.

If you want, you could also provide an upsert option.

[kellrott]

Attempt in #45

[adamstruck]

We seem to have settled on the Get / Add approach for modifying a vertex. This still needs to be described in the documentation.