(gh-11) correct eta and % complete calculations

Prior to this commit, the percentage complete would be wildly
inaccurate, relying on data from the executors and not from
the parent. It would iterate through executors, overwriting
the value each time.

This commit accomplishes two things:
  - Uses the Jenkins API's 'timestamp' and 'estimatedDuration'
    to calculate progress_pct
  - Calculates ETA in minutes and seconds and adds it to the API;
    previously, this calculation lived in an erb template.
Merge pull request #20 from rji/fix-build-progress

(gh-11) correct eta and % complete calculations