More selectors: prev, next, and nearest

`prev` and `next` only search siblings, fairly limited.  `nearest` is awesome, though.  It's roughly based on the **jquery-closest** plugin (https://github.com/fusionbox/jquery-nearest).

It's helpful when you have a bunch of containers that are repeated, so that the `motion_kit_id` of the container view will show up again and again.

```
UIView(id: :row)
  - UILabel(id: :name)
  - UIImageView(id: :avatar)
UIView(id: :row)
  - UILabel(id: :name)
  - UIImageView(id: :avatar)
UIView(id: :row)
  - UILabel(id: :name)
  - UIImageView(id: :avatar)
```

In the context of `:name`, `nearest(:avatar)` will return the expected `UIImageView`.  In fact, that same view will be found as long as it is *anywhere inside :row*.  e.g.

```
UIView(id: :row)
  - UIView
    - UILabel(id: :name)
  - UIView
    - UIImageView(id: :avatar)
```

The "nearest" algorithm searches in this order:

- self
- children
- siblings
- parent view, and it's children
- grandparent, it's children
- etc


Yeah actually this is the one kinda place I'm OK with them.  I'd be fine converting these to guards.