Add skip_npm option to bundle fabric task

This allows for tighting the code->build->fix->rebuild loop
Add a mocha test harness for js testing

Test modules added to the js/test directory will be picked up
automatically and can require any of the source modules bundled into the
app.

Run the tests by opening opentreemap/treemap/js/test/test.html in a
browser or using testem (https://github.com/airportyh/testem).
Merge pull request #879 from jwalgran/topic/js-test-harness

Add a JS test harness