fix(fromEvent): make selector argument optional in fromEvent static method

This makes it consistent with the signature of the implementation.

Author: jeffbcross

src/Observable.ts

@@ -138,7 +138,7 @@ export default class Observable<T> implements CoreOperators<T>  {
   static forkJoin: <T>(...observables: Observable<any>[]) => Observable<T>;
   static from: <T>(iterable: any, scheduler?: Scheduler) => Observable<T>;
   static fromArray: <T>(array: T[], scheduler?: Scheduler) => Observable<T>;
-  static fromEvent: <T>(element: any, eventName: string, selector: (...args:Array<any>) => T) => Observable<T>;
+  static fromEvent: <T>(element: any, eventName: string, selector?: (...args:Array<any>) => T) => Observable<T>;
   static fromEventPattern: <T>(addHandler: (handler:Function)=>void, removeHandler: (handler:Function) => void, selector?: (...args:Array<any>) => T) => Observable<T>;
   static fromPromise: <T>(promise: Promise<T>, scheduler?: Scheduler) => Observable<T>;
   static interval: (interval: number, scheduler?: Scheduler) => Observable<number>;