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' into 2_0_0

Conflicts:
	lib/less/browser.js
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
Merge branch 'master' into 2_0_0

Conflicts:
	lib/less/browser.js
	lib/less/parser.js
Fix tests
remove commented out code now implemented in parser
add browserify (not yet working) and refactor tree nodes to not be dependent on their parent (currently breaks browser build)
move environment to the less namespace