sss/open-union: Data.OpenUnion1 -> Data.OpenUnion

Data.OpenUnion is now the public-facing interface for type-indexed
co-products.

TODO: incorporate ideas from alternative implementations such as
      <http://okmij.org/ftp/Haskell/extensible/TList.hs>
sss/open-union: don't export constructor for Union

resolves #13
sss/open-union: version bump
Merge pull request #27 from bfops/sss/open-union

open union: Data.OpenUnion unified interface