Code completion in Xcode

Hi!

Masonry looks like a very convenient way of managing AL and I would love to use it in my projects. Unfortunately, there is one annoying problem... Xcode struggles to provide any code completion for properties of protocols, and this affects typing constraints code for `MASConstraintMaker` drastically. You would start typing `make.top.e`, but you'd never get a suggestion of `equalTo` :(

I assume you are using AppCode because it behaves properly here. But I wonder if there's anything possible to do here for us, mere mortals? :) 

One possible solution would be replacing `id<MASConstraint>` with an abstract base class, but I suppose it's too much of an API breaking change...