(QENG-1225) Implement uber_build polling and status query.

I realize this still needs some work but wanted to get some eyes on it before doing much more with it. Some questions:
1. Is it okay to do what I've done with the Pkg::Util::Date.timestamp method? This ensures that we get the same `build_date` that was used to generate the dynamic jenkins job, thereby allowing subsequent rake invocations involving the new utility tasks to successfully query the build jobs. I figure this is okay because documentation for this repository recommends using package:implode before running again--is that accurate? Maybe I should re-read the documentation before making this assumption...
2. Which parts of this need spec tests? Are there spec tests for rake tasks? I could probably figure this out with enough digging around but I don't want to do more work than is necessary ;)
3. Are y'all okay with using net/http rather than curl commands? I know @haus mentioned he would have gone with curl commands...
This should be json_pure to avoid compilation nonsense.