Added a Monoid instance for quantities.
Renamed the ToSIBasis type class.
Merge pull request #16 from dmcclean/monoid

Added a Monoid instance for quantities.
Merge pull request #17 from dmcclean/known-dimension

Renamed the ToSIBasis type class.
Defined the hectare.
Defined the acre. This is the international acre to go with the international foot, yard, etc.
Merge pull request #18 from dmcclean/area

Area units