Add SinonJS v1.7.3 (testing framework)

> SinonJS - "Standalone test spies, stubs and mocks for JavaScript. 
> No dependencies, works with any unit testing framework."

See http://sinonjs.org/ and https://github.com/cjohansen/Sinon.JS (1k+ stars & 150+ forks).

It comes in handy when you want to play around with a standalone mock-server. 

v1.7.3 source -- http://sinonjs.org/releases/sinon-1.7.3.js (minified via UglifyJS)