[ibm-aix] Refactor script (#231)

Make the script more readable, mostly by:

- using the Product classes,
- removing the use of functions when unnecessary,
- a little bit of renaming and documentation.

Also add a link to web.archive.org to fetch new oldest versions.

releases/ibm-aix.json

@@ -8,5 +8,11 @@
   "7.2.2": "2017-10-31",
   "7.1.5": "2017-10-31",
   "7.2.1": "2016-11-30",
-  "7.2.0": "2015-12-31"
+  "7.2.0": "2015-12-31",
+  "7.1.4": "2015-12-31",
+  "7.1.3": "2013-11-30",
+  "6.1.9": "2013-11-30",
+  "7.1.2": "2012-11-30",
+  "7.1.1": "2011-10-31",
+  "7.1.0": "2010-09-30"
 }