Added userinfo endpoint (#300)

* Added userinfo endpoint

* Added documentation for  the userinfo endpoint

* Update oauthproxy.go

Co-Authored-By: Dan Bond <pm@danbond.io>

* Suggested fixes :  Streaming json to rw , header set after error check

* Update oauthproxy.go

Co-Authored-By: Dan Bond <pm@danbond.io>

* fix session.Email

* Ported tests and updated changelog