Implement dumpLineNumbers functionality (issue #50)

This implements the dumpLineNumbers env variable to enable source mapping.  It required a fix to less.rb - see https://github.com/cowboyd/less.rb/pull/60.  As such, this references my fork of less.rb. 

My first pull request (second, technically), but I'm assuming it was best to go ahead and push this (for discussion/testing purposes) and that it's easy to yank the reference to my fork when you merge this.  If not, I'm happy to re-push when the less.rb PR gets merged.