allow angular.d.ts to be imported using AMD

currently if you do 
import angular = require("angular"); 
tsc will throw 2071 - Unable to resolve external module 'angular'
this 
declare module 'angular'{
    export var angular:ng.IAngularStatic;
}
declares such a module.
Update angular.d.ts
Merge pull request #2213 from kostaad/patch-1

allow angular.d.ts to be imported using AMD