Change 204 response from MUST to SHOULD
Remove trailing whitespace
Clarify and extend updating of relationships

The following requirements were added to improve the consistency and correctness
of updating relationships:

* A `PUT` request should be allowed to the URL of a to-one relationship
* A `POST` request sent to a to-one relationship URL that already contains a relationship should fail
* A `DELETE` request sent to a to-one relationship URL that doesn't contain a relationship should fail
* A single resource ID (or an array of IDs, as previously stated) can be POSTed to a to-many relationship URL
* 404 and 409 error responses
Merge branch 'gh-pages' of https://github.com/harrison/json-api into clarify-relationship-updates
Merge pull request #279 from dgeb/clarify-relationship-updates

Clarify updating of relationships