Fix conversion to/from Ufixed* types

* Previously, conversions were attempting to force conversion to,
  e.g., HSV{Ufixed8}, which is not a valid type
Merge pull request #63 from JuliaLang/kms/Ufixed_conversion_fixes

Fixes conversion to/from Ufixed* types
Merge branch 'master' into teh/colortype

Conflicts:
	src/colorspaces.jl