Additional request params

Adds the possibility to define additional http request parameters for every ajax request of Request/REST stores.

I'm only eyeballing this, but wouldn't this cause a problem if both `queryParams` and `userParams` are defined?  The last value of `queryParams` and the first key of `userParams` would get smooshed together and basically lead to an invalid query string.  e.g. `queryKey=queryValueuserKey=userValue`