Cleanup sample code.

1. Removed uses of advanced flask features like url_for and blueprints
2. Took out JS_URL from the config and put in AFFIRM_SERVER_NAME so that
   we can re-use it when calling capture/cancel
3. Added API_VERSION to the config - I want people to get used to having
   the API_VERSION defined as a constant so that they can start using a
   new version across the board easily
4. Standardized on double-quotes (replaced instances of single quotes)
5. Implemented capture/cancel upon order notification. Comments describe
   how to verify sku pricing
6. Made INJECT_CHECKOUT_NOTIFICATION=True the default
7. Removed shop.py/webooks.py - everything is now defined in __init__.py