Add label key namespacing

Allow "name.space/key" format.  Also loosen DNS952 to 1035 (limit 24 -> 63). 
I agree this is confusing.

Maybe we can borrow syntax from ```QName``` and just say valid label keys are comprised of two segments - a prefix and name - separated by slash?

http://en.wikipedia.org/wiki/QName

```
Label := PrefixedName | UnprefixedName
PrefixedName	::=   Prefix '/' LocalPart
UnprefixedName  ::=   LocalPart
```