Add JS test harness

Changes:
* Add initial JS tests and tests bundle task (say that 10 times fast) to
  gulp build file
* Add the testem HTML harness during provisioning (development only)
* Add ./scripts/testem.sh to run JS unit tests in a browser window
* Add testem (ci mode) to ./scripts/test.sh to run tests on Jenkins
Please add `sudo_user: nyc-trees` to this task.