Wrap 'URL-Based JSON API > IDs' section to 80 chars

This appears to be the convention that's being used more and more in
sections of the document.
Clarify the role of URLs being used as IDs

Per a [discussion][1] on the best practice of the `id` key within the
URL-based format.

[1]: https://github.com/json-api/json-api/pull/179
Merge pull request #180 from andrewsardone/id-and-urls

Clarify the role of id keys within the URL-based JSON API
Unify `id` requirement-level to SHOULD

The `id` requirement-level was inconsistent between the ID-based and
URL-based formats. Based on a [GitHub discussion][ghd], it seemed like
`id` keys are *usually* a good thing, but there are cases where a hard
requirement would be unnecessary. It seems like the spec has settled on
a simple [recommendation][rec] on the presence of the `id` key.

[ghd]: https://github.com/json-api/json-api/issues/181
[rec]: http://www.ietf.org/rfc/rfc2119.txt
Document rationale for omitting IDs

[Suggested][s] by @dgeb.

[s]: https://github.com/json-api/json-api/issues/181#issuecomment-33149350
Merge pull request #182 from andrewsardone/id-requirement-level

Unify id requirement level across spec
Merge URL and ID styles

Removes ambiguity around the `links` key in documents, which clears
path to merge the URL and ID styles of the spec.

This is discussed in detail in #183.
Add Related Tools Section
- List tools that are not specific to application/vnd.api+json but help in the implemetation.
Merge pull request #186 from guillec/gh-pages

Add Related Tools Section
Merge pull request #185 from cerebris/convergence

Merge URL and ID styles
Merge.