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.
always execute import-once in the same way. Fixes #1898
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
final changelog changes for 1.7.0
1.7.0 Release
Add note about clean css options changing
update readme and rename LESS to Less
Merge branch 'master' into 2_0_0

Conflicts:
	lib/less/import-visitor.js
	lib/less/index.js