Wrote HTTP Method Negotiation chapter
Wrote "Error Reporting" chapter
Wrote "What is an API?" section
Added Representation Formats chapter
HTTP code blocks for requests

- Requests can be rendered as HTTP code blocks as long as the request line ends
  in `HTTP/<version>`
Make all inter-page links relative to project root

- Will simplify linking in tools that generate docs from markdown
Created content negotiation chapter
Added content negotiation to TOC
Wrote content validation chapter
Fixed links to content negotiation chapter from content validation
Wrote versioning chapter
Edits to versioning

- for URI examples, quote in code blocks
- minor grammatical fixes
Created stubs for referenced sections

- Sections that are planned, but not yet written.
Merge branch 'feature/api-primer'

Close #2