Changed the static method to a static getter for a cleaner usage syntax

Author: Schniz

modules/BrowserHistory.js

@@ -108,7 +108,7 @@ class BrowserHistory extends DOMHistory {
     }
   }
 
-  static history() {
+  static get history() {
     instance = instance || new BrowserHistory();
     return instance;
   }

modules/HashHistory.js

@@ -169,7 +169,7 @@ class HashHistory extends DOMHistory {
     return '#' + path;
   }
 
-  static history() {
+  static get history() {
     instance = instance || new HashHistory();
     return instance;
   }

modules/__tests__/BrowserHistory-test.js

@@ -3,5 +3,5 @@ import BrowserHistory, { history } from '../BrowserHistory';
 
 describe('BrowserHistory', function () {
   describeHistory(new BrowserHistory);
-  describeHistory(history());
+  describeHistory(history);
 });

modules/__tests__/HashHistory-test.js

@@ -3,5 +3,5 @@ import HashHistory, { history } from '../HashHistory';
 
 describe('HashHistory', function () {
   describeHistory(new HashHistory);
-  describeHistory(history());
+  describeHistory(history);
 });