Added initial support for video thumbnail fetches

Summary: Added support for Python SDK video thumbnail tests.

Test Plan:
1. upload video assigned to ad account, if not already there.
2. facepy
3. vid = AdVideo(fbid=<VIDEO_ID>)
4. vid.get_thumbnails()

Output of above:
https://phabricator.fb.com/P20051337

Unit tests:
python -m facebookads.test.unit
............................
----------------------------------------------------------------------
Ran 28 tests in 0.015s

OK

Integration tests:
ython -m facebookads.test.integration
....................
----------------------------------------------------------------------
Ran 20 tests in 27.928s

OK

facebookads/mixins.py

@@ -198,6 +198,7 @@ def remove_labels(self, labels=None):
             params={'adlabels': [{'id': label} for label in labels]},
         )
 
+
 class ValidatesFields(object):
     def __setitem__(self, key, value):
         if key not in self.Field.__dict__:

facebookads/objects.py

@@ -1448,33 +1448,31 @@ def get_endpoint(cls):
         return 'adgroups'
 
     def get_ad_creatives(self, fields=None, params=None):
-        """Returns iterator over AdCreative's associated with this ad."""
+        """Returns iterator over AdCreatives associated with this ad."""
         return self.iterate_edge(AdCreative, fields, params)
 
     def get_targeting_description(self, fields=None, params=None):
-        """
-        Returns TargetingDescription object associated with this ad.
-        """
+        """Returns TargetingDescription object associated with this ad."""
         return self.edge_object(TargetingDescription, fields, params)
 
     def get_keyword_stats(self, fields=None, params=None):
-        """Returns iterator over KeywordStats's associated with this ad."""
+        """Returns iterator over KeywordStats associated with this ad."""
         return self.edge_object(KeywordStats, fields, params)
 
     def get_ad_preview(self, fields=None, params=None):
         """Returns AdGroupPreview object associated with this ad."""
         return self.edge_object(AdGroupPreview, fields, params)
 
     def get_reach_estimate(self, fields=None, params=None):
-        """Returns iterator over ReachEstimate's associated with this ad."""
+        """Returns iterator over ReachEstimates associated with this ad."""
         return self.iterate_edge(ReachEstimate, fields, params)
 
     def get_stats(self, fields=None, params=None):
         """Returns AdStats object associated with this ad."""
         return self.edge_object(AdStats, fields, params)
 
     def get_click_tracking_tag(self, fields=None, params=None):
-        """Returns iterator over ClickTrackingTag's associated with this ad."""
+        """Returns iterator over ClickTrackingTags associated with this ad."""
         return self.iterate_edge(ClickTrackingTag, fields, params)
 
     def get_conversion_stats(self, fields=None, params=None):
@@ -1818,6 +1816,28 @@ def waitUntilEncodingReady(self, interval=30, timeout=600):
             timeout,
         )
 
+    def get_thumbnails(self, fields=None, params=None):
+        """
+        Returns all the thumbnails associated with the ad video
+        """
+        return self.iterate_edge(VideoThumbnail, fields, params)
+
+
+class VideoThumbnail(AbstractObject):
+
+    class Field(object):
+        id = 'id'
+        height = 'height'
+        name = 'name'
+        scale = 'scale'
+        uri = 'uri'
+        width = 'width'
+        is_preferred = 'is_preferred'
+
+    @classmethod
+    def get_endpoint(cls):
+        return 'thumbnails'
+
 
 class AdPreview(AbstractObject):