Added a minimum query length constrain to autocomplete

If user types a single chararchter and pauses, the auto-completeion will kick off and fetch a very large number of suggestions, which ofter causes the UI to hang. Therefore I added a option to constrain the minimal length for query strings.
Instead of wrapping everything in a massive if statement, which only makes the indentation harder to read, why not just add, here, something like this:
```javascript
if (textbox.val().trim().length < options.minLength) {
    return;
}
```
Note. I also changed the if statement slightly. `textbox` is a jQuery object so you can use it's `.val()` and by adding a `.trim()` on it we deliberately ignore spaces.