Implement postProcessor callback feature for less.ls browser environment #1835

Add `jasmine:postProcessor` task and corresponding spec as well
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.
Fixed that in some situations a mixin call into a referenced file wouldn't import media queries. Fixes #1469
Fix a small scope issue with mixins when using parent selectors, introduced in 1.6.2. Fixes #1877
when in lint mode, report errors from creating css. Fixes #1786
Do not evaluate css with guards if the guards fail. Fixes #1873
Merge branch 'master' of https://github.com/cloudhead/less.js
tweak the log levels. Fixes #1856
Merge branch 'master' of https://github.com/cloudchen/less.js

Conflicts:
	lib/less/browser.js
add browser details to changelog
add-in more changes to the changelog
call callback if exception whilst calling toCSS. Fixes #1816
re-implement pull request #1701 - still allow log level to be configurable in dev mode
use the right xml http request for local file access. from @Scorpin
allow % to be passed to the unit function. Fixes #1550 and fixes #1571
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.