add rel='canonical' Link header from a resource in a transaction back to the non-transaction version of the resource

Fixes https://www.pivotaltracker.com/story/show/70466198
Correctly format HTTP Link: headers by wrapping the URIs in angle brackets

Fixes https://www.pivotaltracker.com/story/show/70466162
Merge pull request #329 from futures/link-headers

HTTP Link headers