New function xls(): translate a string for JS

yehster wrote on Friday, December 21, 2012:

http://www.open-emr.org/wiki/index.php/Codebase_Security#Plan
Wikipedia page updated to document new xls() function introduced and used for the first time in this commit
https://github.com/openemr/openemr/commit/ec9f238540bb8759ac44abafb238ec9d87139f40