Create an example resource for the variables table.

Based on the work on #188, create a resource that can:

  - [x] Get a list of all the variables in the site.
  - [x] Get the content of a variable (or set of them) by providing the variable name(s).
  - [x] Create a new variable.
  - [x] Update an existing variable.
  - [x] Delete a variable.
  - [x] Explore the enpoints using the `OPTIONS` method.
  - [x] 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?
Lets move this to restful-example module. There could be security implications (we don't check access on set) -- so I want to make it clear it's not part of the core functionality. Maybe also worth also implementing access() method, and allow access only if ``user_access('administer site configuration, $account);``