Create an example resource for the variables table.

Based on the work on #188, create a resource that can:

  - [ ] Get a list of all the variables in the site.
  - [ ] Get the content of a variable (or set of them) by providing the variable name(s).
  - [ ] Create a new variable.
  - [ ] Update an existing variable.
  - [ ] Delete a variable.
  - [ ] Explore the enpoints using the `OPTIONS` method.
  - [ ] Test the endpoints using the `HEAD` method.
 
All the other features (caching, rate limits, authentication, formatting, …) should be granted automatically without any additional work.

@amitaibu does this sound as a good proof of concept for #188?