Merge URL and ID styles

@lgebhardt and I based this PR on our proposal from #183. By removing ambiguity around the `links` key in documents, we were able to merge the URL and ID styles of the spec. In the process of merging these styles, we also tried to clear up any inconsistencies in terminology (e.g. "document" vs. "resource") and layout. I hope we didn't mangle anything.

This is obviously a pretty substantial PR. If it would help, I could create a "convergence" branch on the main repo so we can all work on this together and accept additional PRs before merging to master (er, gh-pages).