Implement function 'replace'

We are using your compiler in the open source project [Stud.IP](http://develop.studip.de) and noticed that your library is missing the LESS string function [replace](http://lesscss.org/functions/#string-functions-replace).

I used the examples from the LESS page to set up the test cases and since it all seems to work fine, I thought I could help making your library more compatible to LESS v1.7.