Introduce support for specifying sort order.

Describes how a server may optionally support requests for
primary and related documents to be sorted according to
specific criteria.
Flip order of to-one & to-many relationship sections

I think it makes sense to start by explaining to-one relationships,
followed by to-many relationships. The simpler case of a to-one
relationship helps build up the format in the reader's mind.
Add to-many format proposed by @mikelinington

Initially proposed in [a comment][ac] within json-api/json-api#183, this
expands the to-many relationship format to accomodate an array of `link`
objects.

[ac]: https://github.com/json-api/json-api/issues/183#issuecomment-33840493
Define "link" objects in Relationships section

Per @dgeb's [suggestion][s].

[s]: https://github.com/json-api/json-api/pull/187#issuecomment-33905900
Remove code-literal wrapping of "link" objects

Per @dgeb's [comment][c], these objects won't appear in the JSON under a
literal `"link"` key, so they shouldn't written as such.

[c]: https://github.com/json-api/json-api/pull/187#issuecomment-33905900
Merge pull request #189 from diosney/gh-pages

FAQ: New Q/A: How discover resource capabilities?
Merge pull request #187 from andrewsardone/to-many-link-objects

Add to-many relationship format as an array of link objects
Merge pull request #148 from dgeb/sorting

Introduce support for specifying sort order.
Fix formatting of SHOULDs & MUSTs in Sorting section
DELETE: Added new semantic: resource collection deletion.