add a demo for one-legged OAuth1 using Noun Project API

[cderv]

This closes #542 by providing a demo on using oauth_signature to use an OAuth1.0a One legged API.

[cderv]

Unfortunately, I am not sure how to do for demo to provide Key and Secret. I think demo files are run at built time, or at least can be run using demo().

Currently I left the Sys.getenv but it won't work unless the user register himself and add the env to Renviron. It this ok ?

[hadley]

Maybe use rstudioapi::askForPassword() instead of Sys.getenv()? Then it's one it's a bit more direct for the user.

You don't need to worry about the demos being automatically rerun.

[hadley]

Would you mind using the standard tidyverse style here?

[hadley]

Actually, let me just finish this off since the changes are small and I need to practice my contributing back to PR skills

[hadley]

Thanks @cderv!

[cderv]

Thanks!
I really need to study and take care of the tidyverse style to stop having reviews about it 😉