BUG: Relative luminance calc in Styler is wrong (and out of date)

[attack68]

The relative luminance calculation is described here: https://www.w3.org/WAI/GL/wiki/Relative_luminance. RL is used by Styler.background_gradient() to determine text-color against background but the calculation is wrongly coded.

In addition a new errata posted suggests that a scalar should be changed (albeit this has minimal impact). The greatest change is had through the re-engineered calculation. See below for before and after.