Added pickle support for Resource classes, albeit a bit hacky.

Unpickling requires the class to be known, so we can't simply use to_dict()
to pickle and pass the dict to __init__() to unpickle. This works for the
pickled object itself (as pickle restores its type), but fails to restore
the class of subobjects, such as some_node.picture. This is why the code
now pickles each subobject too.

tests/test_pickle.py

@@ -0,0 +1,79 @@
+# -*- coding: utf8 -*-
+import pickle
+import unittest
+
+
+class PickleTest(unittest.TestCase):
+    def test_pickling_node(self):
+        from pillarsdk import Node, File
+
+        base_link = 'https://storage.googleapis.com/57534c07c37a7195f/_%2F326760eb7d7b244afe52'
+        picture = File({
+            '_id': '57534ccdc379cf1b24a7196d',
+            '_created': '2016-06-04T23:49:01.000+0200',
+            '_updated': '2017-09-07T15:55:37.000+0200',
+            '_etag': '4bb3f525ea637c612fca882e5fcf334c056544fd',
+            'status': 'complete',
+            'name': '326763a705364fe0b0eb7d7b244afe52.png',
+            'backend': 'gcs',
+            'format': 'png',
+            'variations': [
+                {'width': 160,
+                 'length': 4705,
+                 'content_type': 'image/jpeg',
+                 'height': 160,
+                 'file_path': '326763a705364fe0b0eb7d7b244afe52-b.jpg',
+                 'size': 'b',
+                 'link': base_link + '-b.jpg'},
+                {'width': 269,
+                 'length': 8508,
+                 'content_type': 'image/jpeg',
+                 'height': 269,
+                 'file_path': '326763a705364fe0b0eb7d7b244afe52-h.jpg',
+                 'size': 'h',
+                 'link': base_link + '-h.jpg'},
+                {'width': 269,
+                 'length': 8508,
+                 'content_type': 'image/jpeg',
+                 'height': 269,
+                 'file_path': '326763a705364fe0b0eb7d7b244afe52-m.jpg',
+                 'size': 'm',
+                 'link': base_link + '-m.jpg'},
+            ],
+            'filename': '01d.png',
+            'project': '57534c07c379cf1b24a7195f',
+            'width': 269,
+            'length': 9681,
+            'user': '573dff22c379cf12e649f07a',
+            'content_type': 'image/png',
+            'height': 269,
+            'file_path': '326763a705364fe0b0eb7d7b244afe52.png',
+            'md5': '',
+            'length_aggregate_in_bytes': 47050,
+            'link': base_link + '.png',
+            'link_expires': '2117-09-08T14:54:35.250+0200',
+        }
+        )
+        parent_node = Node({
+            '_id': '54134',
+            'name': 'Dadday',
+        })
+        original = Node({
+            '_id': '123456',
+            'name': 'über cooole node',
+            'parent': parent_node,
+            'picture': picture,
+        })
+
+        pickled = pickle.dumps(original)
+        restored = pickle.loads(pickled)
+
+        self.assertEqual(restored._id, '123456')
+        self.assertEqual(restored.name, 'über cooole node')
+
+        self.assertIsInstance(restored.parent, Node)
+        self.assertEqual(restored.parent.name, 'Dadday')
+
+        url = 'https://storage.googleapis.com/57534c07c37a7195f/_%2F326760eb7d7b244afe52-m.jpg'
+        self.assertEqual(url, original.picture.thumbnail('m'))
+        self.assertEqual(url, restored.picture.thumbnail('m'))