Major recoding

Hi,

I didn't like the fact, that you were stuck with only one thumbnail size. That's why I rewrote some code.
The field now take a variations dictionary like that:
`StdImageField(upload_to='', variations={'thumbnail': (100,100, True), 'small': (800,800), 'extra_large':(1200,1200)})`

There still is legacy support, but it will raise a DeprecatedWarning.

What do you think?