Let `luma` follow spec

The `luma` function found in LESS is not implemented as defined in the specification (http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef).
Let tests reflect output of new luma function.
Add test case were luminance is not equal to luma.
Sync semantics of test variables and their output.
Introduce luminance function.

The behaviour of this function is identical to luma prior to LESS 1.7.
Merge pull request #1890 from roelvanduijnhoven/feature/luma-definition

Let `luma` follow spec