Allow overriding jquery's automatic cache-busting on ajax calls

By default jQuery (and Select2) add parameters to ajax calls, to invalidate browser caching. In certain circumstances its useful to disable this (eg when server sets good Cache-Control headers), this patch allows passing the cache/jsonpCallback params though to jquery's ajax function.
Merge pull request #1186 from barryhunter/patch-1

Allow overriding jquery's automatic cache-busting on ajax calls