Incorrect return type in NgPushRegistration.log()

In the https://github.com/angular/mobile-toolkit/blob/master/service-worker/worker/src/companion/comm.ts we can see that metho `.log()` returns `Observable<string>` wile the return type is in fact different

The output from
```
this.sw.log().subscribe(msg => console.log(msg));
```
is:
![Uploading Screen Shot 2017-07-20 at 8.44.50 AM.png…]()
