Make VersionedKeyValSource validate explicitly provided versions
fix comments
Merge pull request #824 from isnotinvain/alexlevenson/VersinonedKeyValSource

Make VersionedKeyValSource validate explicitly provided versions
Remove answered TODO question from VersionedKeyValSource
Merge pull request #825 from isnotinvain/alexlevenson/VersinonedKeyValSource

Remove answered TODO question from VersionedKeyValSource
Merge https://github.com/twitter/scalding into develop