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

Add `jasmine:postProcessor` task and corresponding spec as well
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