Add support for HTTP response objects to Image.open()

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.

    Image.open(urllib2.urlopen(url))
    Image.open(requests.get(url, stream=True).raw)

PIL/Image.py

@@ -109,6 +109,7 @@ from PIL._util import deferred_error
 
 import os
 import sys
+import io
 
 # type stuff
 import collections
@@ -2248,6 +2249,11 @@ def open(fp, mode="r"):
     else:
         filename = ""
 
+    try:
+        fp.seek(0)
+    except (AttributeError, io.UnsupportedOperation):
+        fp = io.BytesIO(fp.read())
+
     prefix = fp.read(16)
 
     preinit()