Will/jenkins detached head

This PR updates the Jenkins scripts to allow us to collect diff coverage information.

The Jenkins Git plugin checks out a specific commit, not a branch.  When we call `git diff origin/master...HEAD`, we therefore get ALL the changes in master except for the specific commit we checked out.

To solve this, the updated script creates a local branch and sets it up to track the remote branch being tested.

The PR also updates to `diff-cover` version 0.1.1, which uses `git diff origin/master...HEAD` instead of `git diff master...HEAD`.  Since the local `master` branch is not always current in Jenkins, this ensures that we're comparing to the most recent version of `master`.

Suggested reviewers: @jzoldak @cpennington