Require for native APIs

[PanayotCankov]

The global object is currently polluted for iOS with all classes, structs, constants, functions etc. etc. for all iOS SDKs and additional libraries included in an App.

~~We want to expose the native APIs through require. So for example the UIButton will not appear as a class available in the global object but rather should be pulled through require("objc!UIKit").~~ Consider implementing this on top of the SystemJS #165 instead of the current Require implementation.

We will try to keep that backward compatible and preserve the SDK APIs in the global object but we will try to promote the require usage as best practice.

[moll]

Remember to perhaps scope or somehow differentiate the built-in module names of APIs from regular NPMs.