Add HSL support on colors conversions

[NumEricR]

Conversions to implement :

• HSL to Hex
• Hex to HSL
• HSL to RGB
• RGB to HSL

[NumEricR]

Half of this work is now done: all colors can be converted to HSL!

Now I have problems to create conversions from HSL colors, especially with the hue2rgb alogrithm, it doesn't give the real equivalents.
If I enter hsl(210, 13%, 50%), I should get #708090 or rgb(112,128,144) ... but it returns #6f8090 or rgb(111,111,111). However I use the algorithm given in a W3C Recommendation.
Same problem already exists to generate hex value on other tools like CSS.coloratum (see here).

You can check that on my "HSL" branch. See #442 and #447