WIP: Atom feed for /repos/:owner_name/:name/builds

See travis-ci/travis-core#82

TODO: Link to indivisual build.
TODO: Add specs.
TODO: Review `#apply?`
Use here-doc for Atom feed template

Rather than unnaturally re-reading __FILE__ and splitting,
use a here-doc to clarify the intent.
(Should also consider splitting it to a file.)
Construct build result URL from parts

Currently, there is no easy way to grab this information from
the Build model.
So we need to construct it from various parts at our disposal.