Paypal-checkout-components: Example app with demos for each use case

[bluepnume]

Would be good to have a test app with a bunch of examples of all of the new integration patterns:

• [x] Basic Client Integration
• [x] Basic client integration with a confirmation page
• [x] Advanced Server Integration
• [x] V.Zero button
• [x] Different button styles
• [x] Billing agreement button
• [x] Mark integration
• [x] Hybrids of basic and advanced

We should:

• [x] Put this behind a node server so we can do server side REST calls and V.Zero SDK calls
• [x] Put it up at something.paypal.com on a c3 instance.
• [x] Have a split view (like plunkr) with a code editor on one side so people can try making modifications. I'm thinking this: https://ace.c9.io/#nav=about -- but obviously no need for iframes
• [x] Update developer.paypal.com with links for each kind of integration
• [x] Let's keep the source for the example app in this repo, so anyone can run it and see the source if they want to.

[bluepnume]

@abarco added some more details of what I'm thinking.

[bluepnume]

This is all ready to merge now, and going out with the next developer.paypal.com push.

[bluepnume]

Finally live!

https://developer.paypal.com/demo/checkout