Fix for junk JPEG data

 * Possible fix for https://github.com/python-imaging/Pillow/issues/630: skip if first byte isn't 0xFF. See this comment for more details: https://github.com/python-imaging/Pillow/issues/630#issuecomment-42224046. You can see the changes more clearly in the first commit: https://github.com/hugovk/Pillow/commit/e835dd70a1183aaab7adaa43a7f28221038d28c9

 * I tested locally with `im = Image.open(filename)` and `im.show()` for each image from #630.

 * It would be good to a test to run on a problem file. Is this possible?

 * Some pep8 cleanup (the second commit: https://github.com/hugovk/Pillow/commit/f4071ade0a995c02fcbc40ff1f3982bbbda2b4ff).