Fix tests and add missing contains function

Reference to src/extensions.js was missing from SpecRunner.html.

item.quantity was used in a couple of places instead of item.quantity().

contains(sku) function was used in tests, but did not exist in the cart.
Have added it.
Reminify using YUI Compressor

Minified file in the repository was 216KB. Not sure what minification /
obfuscation tool was used previously, but I used YUI Compressor to
reminify to 3KB.
Fix discount which was being applied qty times instead of once

Includes a test to illustrate the problem.