Created explicit export list.
Removed obsolete comment.
Wikipedia reference for imperial cup.
Merge pull request #19 from dmcclean/explicit-exports

Explicit exports
Hide Variants(..), explicity constructors.
Made the Dimension' constructor strict in all its fields.
Added a simplification rule for dimensional division, and a convenience operator for dimensional inversion.
Added US customary units of liquid volume measurement.
Merge pull request #25 from dmcclean/strict-term-level

Made the Dimension' constructor strict in all its fields
Merge pull request #28 from dmcclean/us-volumes

Added US customary units of liquid volume measurement.
Merge pull request #26 from dmcclean/inverse

Added a convenience operator for dimensional inversion
Port to NumType-DK 0.3.
Redo Show per Doug\'s comments on 1134a98
Export siUnit instead of Dimensional constructor.
Merge changes from bjornbm/master.