Added proxy object for AppKitBindings

:warning: **Do Not Merge** :warning: work in progress of #452

I have the basic idea of it working. Doing either of these works fine:

```objc
RACBinding *binding = [self.textField1 rac_bind:NSValueBinding nilValue:@""];

// 1. use it as a signal
[binding subscribeNext:^(id x) {
    self.textField2.stringValue = x;
}];
// 2. use it as a subscriber
[[self.textField2 rac_textSignal] subscribe:binding];
```

However, `bindTo:` doesn't work properly yet.
Also the tests aren't correct yet.

## Questions

- How do you add a new test file to the build? I added it to the build sources, but it isn't been tested.
- Are any references leaked?