Update build.

This includes:
1. Update browserify version.
  - update keymaster for browserify patch, and move the require from file.js to app.js
  - manually set Backbone.$ = $ (needed due to browserify changes)

2. Use browserify-shim to externalize chai dependency in tests.

3. Simplify gulpfile code.
Build out test framework to support testing Backbone.

Add Sinon (for mocking the server and spy functions), and SinonChai (for nice assertions on Sinon spies).
build: add support for watching/rebuilding test files, and for re-running tests on changes.
Merge pull request #794 from prose/testing-setup

Core testing setup to allow tests for the Backbone app.  (Includes dependency update; remember to npm install.)