Improve examples, add more tutorials, and review existing tutorials

[loongy]

All examples could be improved by:

1. Providing a clonable repo of the example
2. Providing a CodePen (or similar) with the code of the example
3. Providing a link to the deployed version of the example (whether its contracts or a UI)

An example on how to integrate to import and deploy the darknode-sol contracts in a truffle project should should be written.