Add: thumbnail helper function

Hi,

I added thumbnail helper function so that if someone simply want to create thumbnails he don't need to reinvent the wheel every time.

I have one question about behaviour:
* Python PIL thumbnail method process images even if they are smaller than max provided sizes (resize them to same size) and don't process images only if file sizes are same as max provided sizes.
* I made this method to not process images if they sizes are same or smaller than max provided sizes.

Which approach do you prefer?

Best regards,
Dobrosław Żybort