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!
Based on the method or link relation, it can be safe or unsafe. The URL does not determine it's safety. Either method defines if it's safe or not, or the case could be made that the link relation defines the safety of it (like ALPS, how it defines safety for link relations). I'm still undecided on that.

In other words (a question for @fosrias), does an unsafe link relation in ALPS doc mean the link is unsafe even it uses `GET` at runtime? Or does it mean an unsafe link relation MUST use an unsafe HTTP method (if over HTTP)?