Merge pull request #4 from Ferada/hex-colors

Add some test cases for parsing/printing hex colors.
Minor corrections.

1. renamed hex-colors.lisp to hexcolors.lisp, consistent with the system
definition

2. unknown :stream keyword replaced by :destination.
Extracted future shared NORMALIZED-HUE.
Added HSL class.
Added conversion RGB->HSL->RGB.
Added default when missing direct conversion.
Generic conversion operations for HSL.
Add HSL combination.
Added lightness calculations.
Add tests for HSL and lightness calculations.