Add CodePen examples

[micahalcorn]

This resolves #11 by adding three examples with code playgrounds:

1. Get listing
2. Get listing by index
3. Create listing

This is a minimal proof-of-concept, which only handles the failure case of not having a web3-enabled browser. I will create additional GitHub issues for other enhancements.

[wanderingstan]

Noticed that the examples fail silently if you're not on Rinkeby. I suggest we move that test, and the test for web3 in general, to a standard js file that's included with codepen.

But it still looks great as is. I'll just make an issue for the Rinkeby thing and merge+deploy this PR.

[wanderingstan]

Also, looks like we can style our pens to match the colors of our docs site:
https://blog.codepen.io/documentation/features/embedded-pens/#embed-themes-1

@micahalcorn can you pm me the OriginProtocol login creds?

EDIT: Looks like its pretty limited what we can do, but we can still at least match the header to our colors.