Allow to pass user headers only (issue #205)

* This fixes https://github.com/bitly/oauth2_proxy/issues/205
* Add new boolean option -pass-user-headers
  to control whether X-Forwarded-User and X-Forwarded-Email
  headers will be set (as opposed to HTTP BASIC auth)
* This is required e.g. for grafana [1] where
  X-Forwarded-User is needed but HTTP BASIC auth fails
  (password is not known and must not be known in this scenario)
* Keep behaviour of PassBasicAuth unchanged for compatibility

[1] http://docs.grafana.org/installation/configuration/#authproxy

oauthproxy_test.go

@@ -156,6 +156,7 @@ func TestBasicAuthPassword(t *testing.T) {
 	opts.ClientSecret = "foobar"
 	opts.CookieSecure = false
 	opts.PassBasicAuth = true
+	opts.PassUserHeaders = true
 	opts.BasicAuthPassword = "This is a secure password"
 	opts.Validate()