[author] Add xls 0.6.4

xls.js grabbed from tag v0.6.4:
https://github.com/SheetJS/js-xls/blob/v0.6.4/xls.js

Minified version generated using uglifyjs 2.4.12:
$ uglifyjs xls.js > xls.min.js
Merge pull request #2591 from SheetJS/master

[author] Add xls 0.6.4