Adding the missing func page.replace() to type def

According to the source code, this replace function exists but it is missing from our type definition. Please promptly merge. I've tested this on my local machine.
https://github.com/visionmedia/page.js/blob/master/page.js#L272

page/page.d.ts

@@ -86,6 +86,11 @@ declare namespace PageJS {
          *
          */
         redirect(page: string): void;
+        /**
+         * Replace `path` with optional `state` object.
+         *
+         */
+        replace(path: string, state?: any, init?: boolean, dispatch?: boolean): Context;
         /**
          *  Navigate to the given path.
          *