Allow saving of large progressive jpg images.

Large jpg images with progressive=True as an image option need an increased buffer to be able to save without an IOError. See http://stackoverflow.com/questions/6788398/how-to-save-progressive-jpeg-using-python-pil-1-1-7
Add MAXBLOCK limit and progressive check
Checks to ensure other IOErrors aren't hidden.

- if the original MAXBLOCK is more than 1MB, raise the exception
- ensure the original MAXBLOCK is reset, even if exception is raised on saving
Merge pull request #3 from brew/patch-1

Allow saving of large progressive jpg images.