Elements of a Hypermedia Message Document

Per issue #4, this is the first draft of this document describing the elements of a hypermedia message. It is all open for discussion, and I'd be happy to change anything. Below are some discussion items.

- [x] 1. Is the categorization of transitions helpful?
- [x] 2. Should this include the concept of curies or prefixes?
- [x] 3. Is the concept of anonymous links and actions helpful?
- [x] 4. Does it make sense to include embedded resources under transitions?
- [x] 5. How do we address more complex attributes? Ones that may include arrays of objects in JSON or nested elements in XML?

Once we come to a consensus, we can merge it into the repo. Thanks!
In addition I would explicitly state that this document is describing elements of Hypermedia Messages in HTTP APIs, not in general.