Offer examples based on CRI wiki

[cderv]

There are a lot of example in CRI wiki.

The aim is to be able to reproduce them all (more or less) using crrri.

The way to do it is still to determine. Ideas:

• Another repo like knitr examples
• A demo folder, from R package structure, and as httr
• Vignettes - but with a workaround to use promises
• just a folder in the github repo, ignore by .Rbuildignore

We'll begin with a choice easy to change if needed.

[cderv]

I think i'll work again on that with the new API and it could be a good way to add functional tests.