Define async APIs and switch Job to use HttpEngine directly.

Using HttpEngine directly introduces some duplicated code with
HttpURLConnection. It also breaks the response cache. I think
this is the best route going forward; and eventually we could
invert this relationship to have HttpURLConnection depending on
Job directly rather than vice versa.
Merge pull request #377 from square/jwilson/async_apis

Define async APIs and switch Job to use HttpEngine directly.