New subtype unifier type class

Added a `SubtypeUnifier` type class to support the kind of normalization discussed [here](https://groups.google.com/d/msg/shapeless-dev/HvV63toSBNM/Nv0DsuBLJzEJ).

Also added a constraint to one of the implicits that creates `FilterAux` instances to avoid creating invalid instances like this:

```scala
implicitly[FilterAux[Foo :: Foo :: HNil, Foo, Foo :: HNil]]
```

Together these two changes allow us to solve [this problem](https://groups.google.com/d/msg/shapeless-dev/HvV63toSBNM/K84lmwSsYzwJ) very concisely: 

```scala
import shapeless._

implicit class Uniqueable[L <: HList](l: L) {
  def unique[A](implicit ev: FilterAux[L, A, A :: HNil]) = ev(l).head
}

class Foo; class Bar extends Foo

// These two compile:
(1 :: new Bar :: new Foo :: "a" :: HNil).unique[Bar]
(1 :: new Bar :: new Foo :: "a" :: HNil).unique[Foo]
```
But if we want to be sure that there's a unique `Foo` (including subtypes of `Foo`), we can write the following:

```scala
(1 :: new Bar :: new Foo :: "a" :: HNil).unifySubtypes[Foo].unique[Foo]
```

And we'll get a compiler error, as desired.