Add support for getting user id

For #1090

lib/browser/user.js

@@ -35,7 +35,8 @@ export default class User {
 
 createMethods(User.prototype, objectTypes.USER, [
     'logout',
-    '_sessionForOnDiskPath'
+    '_sessionForOnDiskPath',
+    'retrieveAccount'
 ]);
 
 export function createUser(realmId, info) {

lib/user-methods.js

@@ -229,6 +229,29 @@ module.exports = {
                     url: url.href
                 }
             });
-        }
+        },
-    }
+        retrieveAccount(provider, provider_id) {
+            checkTypes(arguments, ['string', 'string']);
+            
+            let url = url_parse(this.server);
+            url.set('pathname', `/api/providers/${provider}/accounts/${provider_id}`);
+            let headers = {
+                Authorization : this.token
+            };
+
+            const options = {
+                method: 'GET',
+                headers: headers,
+                open_timeout: 5000
+            };
+            return performFetch(url.href, options)
+                .then((response) => { 
+                    if (response.status !== 200) {
+                        return response.json().then((body) => callback(new AuthError(body)));
+                    } else {
+                        return response.json();
+                    }
+                });
+        },
+    },
 };