JH 2 - Basic N2 User subsystem extensions to support Identity

Proposed N2 extensions are introduced to support Asp.Net Identity: new class UserExternalLoginInfo is proposed to store member's external login provider info (Facebook, Google etc).
ItemBridge gets additional methods to support new class.
Proposed changes have no impact on existing N2 logics.

ItemBridge gets some comments on known open issue that may be a security risk.