Add ability to wrap response in an envelope

Thanks to @Japh on #507 for the original patch, and to WP.com for
inspiration.
Merge branch 'develop' into two-http-envelope
Ensure envelope_response is public
Add unit test for enveloping support
Merge pull request #628 from WP-API/two-http-envelope

Add ability to wrap response in an envelope
Merge branch 'develop' into two-posts-routes