added background_image filter

tweaked the background filter:

### The `background` filter

The background filter can now create a background-image with dimensions and color, default is white (#FFF). where you can put other images/filters ontop

Configuration looks like this:

``` yaml
liip_imagine:
    filter_sets:
        my_thumb:
            filters:
                background: { size: [1026, 684], color: '#fff' }