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
added gradle based test suite, reviewed rhino integration
added gradle based test suite, reviewed rhino integration
merged upstream/master
normalize quotes to apostrophes in error messages from javascript evaluation (unify node/rhino error message)
fixed some jshint errors
removed Makefile (again), fixed grunt rhino build
renamed task cleanupTest to clean
added gradle wrapper scripts
fixed test for initRhinoTest function
Add visitor.js array-based function cache for faster lookups. perf / #1615
Optimize visitor.visitArray() for perf. #1615
Misc. perf optimizations. #1615
New input chunker in parser.js. #1615
Minor fixes. #1615
renamed path implementation in less for rhino to _path