Add a method to the js sdk to look up 3pids on the ID server.

lib/client.js

@@ -3233,12 +3233,29 @@ MatrixClient.prototype.requestEmailToken = function(email, clientSecret,
     return this._http.idServerRequest(
         callback, "POST", "/validate/email/requestToken",
         params, httpApi.PREFIX_IDENTITY_V1
-    ).then(function(res) {
-        if (typeof res === "string") {
-            return JSON.parse(res);
-        }
-        return res;
-    });
+    );
+};
+
+/**
+ * Looks up the public Matrix ID mapping for a given 3rd party
+ * identifier from the Identity Server
+ * @param {string} medium The medium of the threepid, eg. 'email'
+ * @param {string} address The textual address of the threepid
+ * @param {module:client.callback} callback Optional.
+ * @return {module:client.Promise} Resolves: A threepid mapping
+ *                                 object or the empty object if no mapping
+ *                                 exists
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ */
+MatrixClient.prototype.lookupThreePid = function(medium, address, callback) {
+    var params = {
+        medium: medium,
+        address: address,
+    };
+    return this._http.idServerRequest(
+        callback, "GET", "/lookup",
+        params, httpApi.PREFIX_IDENTITY_V1
+    );
 };
 
 /**

lib/http-api.js

@@ -245,7 +245,12 @@ module.exports.MatrixHttpApi.prototype = {
             opts,
             requestCallback(defer, callback, this.opts.onlyData)
         );
-        return defer.promise;
+        // ID server does not always take JSON, so we can't use requests' 'json'
+        // option as we do with the home server, but it does return JSON, so
+        // parse it manually
+        return defer.promise.then(function(response) {
+            return JSON.parse(response);
+        });
     },
 
     /**