Merge the list of objects into one and include a type attribute

A pull request where we can discuss whether or not to use a `type` property in a generic `objects` array versus separate lists of `nodes`, `ways`, and `relations`.

I've created a gist with an example [before this change](https://gist.github.com/iandees/fad0112f0688c11dae38#file-option_a-json) and [after this change](https://gist.github.com/iandees/fad0112f0688c11dae38#file-option_b-json).