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
Adding gsub function
Adding gsub function
Adding capture group test case
Merging remote
proposed solution to #1568: percentage as attribute
util.error is deprecated
Added `length` function (#1542). Added scalar value handling for `extract` and `length` (#1576).
Merge branch 'next'
import multiple has a cascade effect. fixes #1497
Merge branch 'master' of https://github.com/cloudhead/less.js
Merge pull request #1572 from robocoder/patch-2

util.error is deprecated
Merge branch 'master' of https://github.com/seven-phases-max/less.js
Update changelog
Merge pull request #1570 from MSamman/Percent_As_Attribute

proposed solution to #1568: percentage as attribute
1.5.0 beta 4 release - last one I hope
Fix missing parameter to Quoted - Fixes #1562
Fix bad json - extra ,
Fix duplicate selectors sometimes generated in extends. Fixes #1591. Fixes #1581
support nested selectors in extends. part fixes #1580