Add API to delete threepid

Uses https://github.com/matrix-org/synapse/pull/1714

lib/base-apis.js

@@ -712,6 +712,24 @@ MatrixBaseApis.prototype.addThreePid = function(creds, bind, callback) {
     );
 };
 
+/**
+ * @param {string} medium The threepid medium (eg. 'email')
+ * @param {string} address The threepid address (eg. 'bob@example.com')
+ * @return {module:client.Promise} Resolves: The server response on success
+ *     (generally the empty JSON object)
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ */
+MatrixBaseApis.prototype.deleteThreePid = function(medium, address) {
+    var path = "/account/3pid/delete";
+    var data = {
+        'medium': medium,
+        'address': address
+    };
+    return this._http.authedRequestWithPrefix(
+        undefined, "POST", path, null, data, httpApi.PREFIX_UNSTABLE
+    );
+};
+
 /**
  * Make a request to change your password.
  * @param {Object} authDict