Added support for use callback function instead of dictionary in
options.params. This need for dynamically set custom Ajax parameters.

Example:

$(el).autocomplete({
    params : function() {
        return {
            foo : this.dataset.foo,
            bar : this.dataset.bar,
            baz : this.id,
            val : this.value
            ... etc. as you wish ...
        };
    }
});