Adding API calls for helping with selection.

Not sure if anyone else would find this useful; but I wanted to have the quick ability to change the selection on the image. The following commits make selecting the maximum space on an image and moving a selection to be stuck on certain spots extremely quick and easy.  I'm not 100% sure that the code is placed in the correct location of the code-base, but I couldn't find a better spot for it.

Adds the following API methods

* selectMax
* selectCenter
* selectTopLeft
* selectTopRight
* selectBottomLeft
* selectBottomRight

An example usage is the following

```javascript
$(document).ready(function(){
    var jCropApi;
    $('#TheImage').Jcrop({
        aspectRatio: 2 / 3,
        onChange: showCoords
    }, function () {
        jCropApi = this;

        $('button#SelectMax').click(function() {jCropApi.selectMax();});
        $('button#SelectCenter').click(jCropApi.selectCenter);
        $('button#SelectTopLeft').click(jCropApi.selectTopLeft);
        $('button#SelectTopRight').click(jCropApi.selectTopRight);
        $('button#SelectBottomLeft').click(jCropApi.selectBottomLeft);
        $('button#SelectBottomRight').click(jCropApi.selectBottomRight);
    });
});
```