Initial draft of hypermedia elements document

This is a rough draft and skeleton of the hypermedia elements doc. Each point will be expanded in following commits.
Typo fixes
Add more availalbe attributes for transitions
Add quote from URI template RFC
Added language attribute for transitions
Add condition to action for meta data
Add clarity to link and action headings
Fix typo
Introduce idea of safe and unsafe aspects
Add clarity on unsafe transition description
Fix typo
Make input section
Add clarity to input section
Fix typo
Add clarity for link requirements
Clean up and add clarity
Change wording to focus on media types
Provide clarity and make protocol-independent
Merge pull request #10 from smizell/master

Elements of a Hypermedia Message Document
Merge branch 'master' into update_overview