added promise chaining for http

the type of variable num in below sample was not detected without this signature. 
```
var $http: ng.IHttpService;
declare function foo(): ng.IHttpPromise<number>;
declare function bar(): ng.IHttpPromise<string>;
$http.get('asdfsadf').then((resp) => foo()).then((bla) => bla.data).then((num) => num);
```