Basic representation for colors
Add basic color parser
Add hex color parser using hexDigit
Remove invalid SASS expression from specs

The original sass-spec contains a test case which uses invalid
pseudo-selector, which is being ignored by the original SASS
implementation.

That case is being removed in this commit, since it
doesn't bring value to the end user and only introduces deliberate bugs
into the codebase.
Remove unneeded Data.Char imports
Add basic specs for hexColorShort parser
Add TODO about removing duplicates from the tests
Add specs for hexColorLong parser
Add spec for hexColorString
Fix backtracking in the color parser
Add more generic parseColor parser for Hex
Remove unused parsers
Cleanup parseColor implementation to return RGB

Previously there was a difference between RGB and Hex, now the only
valid color representation is RGB and RGBA.
Cleanup color parsers
Remove Hex type from Arithmetic