Adding dimensions as an option for thumbnail and list view.

This adds an option under Settings->Show that lets you display dimensions for images. If not an image then it shows nothing. Screenshot of it below.

![kcfinder](https://cloud.githubusercontent.com/assets/23062/5283058/de03d4fe-7ad0-11e4-97b5-084de166d28c.png)

Size is already extracted on line 717:
```
$size = $img->getSize($file);
```
There is no need to extract the size twice. This can be avoided by replacing your line 740 with:
```
if ($type && count($size) >= 2)
  list($width, $height) = $size;
```