Remove /by-md5/ prefix from image paths

Use forked django-images for now, until we can communicate with the
upstream about it. Fixes #4

pinry/core/utils.py

@@ -0,0 +1,16 @@
+import hashlib
+import os
+
+def upload_path(instance, filename, **kwargs):
+    hasher = hashlib.md5()
+    for chunk in instance.image.chunks():
+        hasher.update(chunk)
+    hash = hasher.hexdigest()
+    base, ext = os.path.splitext(filename)
+    return '%(first)s/%(second)s/%(hash)s/%(base)s%(ext)s' % {
+        'first': hash[0],
+        'second': hash[1],
+        'hash': hash,
+        'base': base,
+        'ext': ext,
+    }