Make using custom Dependency Resolvers way easier

Right now we have a method on top of `IMutableDependencyResolver` that will fill in the RxUI framework classes. This is great for people using `ModernDependencyResolver`, but most dependency resolvers *don't* implement Register the way we do, so trying to get the default dependencies registered on IoC containers like MEF or NInject is a royal pain and completely non-obvious.

This PR makes it way easier, by adding a method to `RxApp`. Now this can be as simple as (for NInject):

```cs
var kernel = new StandardKernel();

RxApp.InitializeCustomServiceLocator((obj, type) => kernel.Bind(type).ToConstant(obj));
RxApp.DependencyResolver = new FuncDependencyResolver(
    (type, contract) => kernel.GetAll(type, contract));
```