Fix rounded colors like ruby sass

Fix lighten/darken/saturate/desaturate functions to work like ruby sass, particularly in limits (very light or dark colors).

Tests add to sass-spec by hcatlin/sass-spec#6


Nevertheless, the sass beahviour can seems quite "strange", because darken(lighten(white,10%),10%) do not give a "white" color but a gray one. So, if you prefer the symetric behaviour, you can also merge only the first commit of this pll request (I'll do an other pull request to simplify you the action)