feat(history): add getAbsoluteRoot method

Per aurelia/router#88 we want to be able to generate an absolute URI with the `router.generate()` method. That method relies upon the history module. `getAbsoluteRoot` allows the router to generate a fully-qualified root URL.

src/index.js

@@ -33,6 +33,13 @@ export class History {
     throw new Error('History must implement deactivate().');
   }
 
+  /**
+   * Returns the fully-qualified root of the current history object.
+   */
+  getAbsoluteRoot(): string {
+    throw new Error('History must implement getAbsoluteRoot().');
+  }
+
   /**
    * Causes a history navigation to occur.
    *