How should we support creating multiple Resources in one request?

Three important use cases for WordPress.com:
- Assign Categories and Tags when creating a new Post.
- Upload an Attachment when creating a New Post.
- Create and insert a `[gallery]` shortcode when creating a new Post.

Speaking in abstract terms, we'll need some standard pattern for creating multiple Resources in one Request.

What prior art can we look to? Good ideas?

Related #559
