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
This should be [handled](https://github.com/azavea/nyc-trees/blob/develop/deployment/ansible/roles/nyc-trees.app/tasks/static-files.yml#L37) by Ansible already.