Added PasteFilterLoader

Added a filter loader for the 'Paste' filter. I really think there ought to be loaders for all the in-built Imagine filters so everything works out the box.

Example configuration for adding a watermark to an image:

    liip_imagine:
        filter_sets:
            product_image:
                quality: 90
                filters:
                    paste: { start: [577, 375], image: web/img/watermark.png }