Progressive JPG support

Sample Usage:

``` py
THUMBNAIL_ALIASES = {
    'app.Model.field': {
        'large': {
            'size': (1920, 0),
            'quality': 85,
            'progressive': True,
        },
        'medium': {
            'size': (600, 0),
            'quality': 85,
            'upscale': True,
        },
        'small': {
            'size': (250, 0),
            'quality': 85,
        },
    },
}
```