LinkedIn Provider

Focused on a design that isolates changes to the new subclass, guarding
from impact to other providers or complicating the Provider superclass.

Improvements

1. json response preference indicated via header instead of querystring
2. xml to json key conversion for claims matching via extension method
3. basicprofile + email for default scope instead of full to fulfill simpler authentication purposes
4. SupportedClaimTypes translated to field selectors when getting Profile Claims.