HTTP response objects returned from `urllib2.urlopen(url)` or
`requests.get(url, stream=True).raw` are 'file-like' but do not
support `.seek()` operations. As a result PIL is unable to
open them as images, requiring a wrap in `cStringIO` or `BytesIO`.
This commit adds this functionality to `Image.open()` by way of
an `.seek(0)` check and catch on exception
`AttributeError` or `io.UnsupportedOperation`. If this is caught
we attempt to wrap the object using `io.BytesIO` (which will
only work on buffer-file-like objects).
This allows opening of files using both `urllib2` and `requests`, e.g.