Refactor and add tests

I've made this handler into a cookbook that can easily be added to any runlist (or other cookbook). In addition, I've added a set of unit tests for the handler itself, and a set of test-kitchen integration tests. I've also updated the handler with a few other changes:

- The `SaladJenkinsTracking` reference in the handler has been removed, because there was no gem dependency for it
- The date rendering from this handler didn't match what Java expected in Jenkins (I should add that the Jenkins side of this uses a deprecated method of parsing dates)
- The MD5 calculation now reads data in 1 MB chunks rather than all at once, which is better for large files
- Changed the method for submitting data to Jenkins so that it renders the JSON cleanly and there are no Chef authorization headers being sent with the request
- Added a `dryrun` attribute so that you can preview the data that's being sent without actually sending it.