Replace `id' return type with `instancetype'

Hey, great library!

I noticed you return `id` on all your class methods. The trouble with doing this is that this sort of thing typechecks:

    NSArray *array = [MLPSpotlight spotlightWithFrame:CGRectZero withSpotlightAtPoint:CGPointZero];

The way to fix this is to use `instancetype` rather than `id`.

You can use return type `id` on `-init` methods because the compiler infers `instancetype` by convention. But the compiler can't infer `instancetype` on your own custom class methods (yet!). Here's an article on the subject:

http://nshipster.com/instancetype/

Thanks!