Adding JImage::cropResize

A lot of setup is required if you would like to both resize an image down to something more manageable, and then crop that created image from the center. This is very useful for creating thumbnails that are both cropped and resized.

You can achieve a similar result using `createThumbs($sizes, JImage::SCALE_FILL)`, but it squashes the image to fit, resulting in a skewed image. See attached imageFill.png for what this looks like.
![imageFill](https://f.cloud.github.com/assets/718028/7057/75c4445a-43e6-11e2-8539-8e6c08536c8e.png)

This method gives a clean resize and crop from center. See attached cropResize.png.
![cropResize](https://f.cloud.github.com/assets/718028/7060/87f910ec-43e6-11e2-868b-a864381bf94f.png)