Make bits serialization sorted (alphabetically)

Yang specification describes canonical form for bits to be ordered by position: http://tools.ietf.org/html/rfc6020#section-9.7.3

But yang-data-api specifies:
public interface BitsCodec<T> extends Codec<T, Set<String>>{
and Set loses the ordering.
This makes ODL report bits in pseudorandom order, which is not nice as users need more logic to compare outputs.

A simple improvement would be to order serialized values alphabetically. It would take some CPU cycles, but most Yang models only have a few bits to sort.