added code to disable caching for ajax requests
appends the no-cache param w/ a timestamp
e.g. ?_=1359063016071
added test cases for disabling cache on ajax queries

- no parameter
- w/ one parameter
- w/ no-cache parameter
- w/ one parameter, then no-cache parameter (replace)
- w/ no-cache parameter (replace), then one parameter
- w/ one parameter, then no-cache parameter (replace), then another parameter