HSLRGB::RGB: Add specs for ::valid_rgb?
HSLRGB::RGB: Fix ::valid_rgb? for more strictly

NOTE: Range#include? depends element#succ on Ruby1.9+ for many objects.
      ::byte? checks Integer. And Range#cover? is more clealy at here.
HSLRGB::HSL: Add specs for ::valid_hsl?
HSLRGB::HSL: Fix ::valid_hsl? for more strictly
Merge pull request #3 from kachick/improve-strictly-validations

Some suggestions for strict validations