Add failing test for imported mixin with wrong path

When relative URLs are on, and there's an image referenced via url(...),
if the URL is unquoted, the generated CSS duplicates the base path
Remove alpha from contrast calc, see #1724
fix the benchmark less
update the changelog with details of the performance improvements
normalize the base path as well as the filename for sourcemaps. Fixes #1736
Merge branch 'master' of https://github.com/cloudhead/less.js
Merge branch 'unquoted-relative-url-in-mixin' of https://github.com/bdukes/less.js
Fix issue with urls in mixins
Merge pull request #1733 from Synchro/master

Remove alpha from contrast calc
Fix issue with unqouted urls in mixins having their context evaluated twice. Fix sourcemap test output
changelog
Merge branch 'master' of https://github.com/cloudhead/less.js