Merge pull request #154 from Eric89GXL/assert-in

ENH: Backport assert_in
WIP: Colors
ENH: Better case support and property use
FIX: Better cases
ENH: Add hsv
ENH: Add Lab space
STY: Clearer methods
FIX: Fix alpha insertion
FIX: Fixed test
ENH: Better conversion
ENH: Add basic linear gradients
FIX: Suggested fixes
STY: Fix
ENH: Add CSS colors
STY: Fix comment
ENH: Add color.constants defs, change darker to match Qt