Implements new AuthorizeAttribute API designed in #956.

This gets rid of the old Mode parameter and replaces it with a
RequireOutgoing parameter.

The AuthorizeModule now passes in an appliesToMethod parameter to
AuthorizeHubMethodInvocation so the AuthorizeAttribute can throw if
it is applied to a method with RequireOutgoing set to true.
Unseal AuthorizeAttribute

#1050
Moved AuthorizeAttribute to the root namespace and folder.