Support for LinkedIn with OAuth2 authentication

Added support for LinkedIn and its OAuth2 implementation.

LinkedIn has its own subtle interpretation of the OAuth2 standard, and requires a few tricks to work.
It uses POST methods and requires all parameters to be sent as body parameters, and ignores query parameters.
It requires some Http headers like Accept to be defined, so to introduce those parameters I used RequestTuner and changed the signature of OAuthService to accept a variable list of RequestTuners. That new signature should change nothing to users of OAuthService interface, but it will break implementors of that interface or of its concrete classes. Hopefully, that means not many people.

Ludovic