Added a look_up_address convenience method

Hard to test since you have to wait for the response, but it works in
the REPL. The API is this:

```ruby
look_up_address address: "1 Infinite Loop" do |points, error|
  # do something with points which is an array
end
```

Suggestions welcome -- thought this might be a good abstraction to have but I'm not stuck on it.

cc @markrickert