Get HList typeclass instances for free with ProductTypeClass

Thanks to #62 I noticed that, beyond not getting a free `TC[HNil]` for having defined `ProductTypeClass[TC]`, typeclass derivation was no longer too happy about deriving the cons parts either:

```scala
[warn] shapeless-contrib/trunk/scalaz/test/scala/LensTest.scala:42:
Attempting to derive a type class instance for class `::`
with sealed superclass `HList`; this is most likely unintended.
To silence this warning, import `TypeClass.ignoreParent`
```

So here they are sitting in `HList`'s companion; they seem to be happy there.