IDM-06: Login to a science gateway with an XSEDE identity

Executive Summary: 
An XSEDE user needs to be able to login to a science gateway (a non-XSEDE web application) using their XSEDE identity, such that the science gateway can securely interact with XSEDE services on behalf of the user.
User Importance Summary: 
We need a way for the XSEDE system to identify individual human beings. If science gateways leverage the same mechanism, it becomes easier for researchers to develop science gateways and it becomes easier for gateway users to access other XSEDE resources and services.
Target Communities and Sizes: 
Gateway developers - 10 < N < 99 People who use gateways - 10,000 < N < 99,999