Oauth2 support for LinkedIn with JSON

Started writing OAuth2 support for LinkedIn and seemed a logical to implement with JSON as the [Spec](http://tools.ietf.org/html/rfc6749Spec) pretty much exclusively mentions that format. It won't break backwards compat as OAuth support will handle all existing cases. The main difference is the response keys are camelCase and the numbers are .. well numbers, so bit rubbish got _EXTRA_DATA.