Add onMove callback

I added "onMove" callback option.

User can set onMove callback function. 
onMove callback runs  when suggested item changed with moveDown() and moveUp().

sample:
```
var autocomplete = $(input).autocomplete({
    lookup: [
                  { value: 'AB', data: 'A1' } ,
                  { value: 'AC', data: 'A2' }
            ],
    triggerSelectOnValidInput: false,
    onMove: function (suggestion) {
                  context = this;
                  value = suggestion.value;
                  data = suggestion.data;
             }
}).autocomplete();
```

I use this feature in addition to "onSelect" in order to measure the usage of function suggest.