[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

ajax/libs/xls/package.json

@@ -0,0 +1,15 @@
+{
+    "name": "xls",
+    "filename": "xls.js",
+    "version": "0.6.4",
+    "description": "Javascript XLS (Excel 97-2004) parser",
+    "homepage": "https://github.com/SheetJS/js-xls",
+    "keywords": ["excel", "spreadsheet", "xls", "parser"],
+    "author": "SheetJS",
+    "repositories": [
+        {
+            "type": "git",
+            "url": "https://github.com/SheetJS/js-xls.git"
+        }
+    ]
+}