[pinterest] implement oauth:pinterest (#83)

Pinterest access tokens are rate limited at 200 requests per
hour (or maybe per 2 or 3 hours?) so having just one access token
for all users isn't going to work in the long run.

test/test_results.py

@@ -40,6 +40,8 @@ class TestExtractorResults(unittest.TestCase):
         config.set(("extractor", "deviantart", "client-id"), "7777")
         config.set(("extractor", "deviantart", "client-secret"),
                    "ff14994c744d9208e5caeec7aab4a026")
+        config.set(("extractor", "pinterest", "access-token"),
+                   "Ab1gUJFF5TFoWXRbX0p7_ue7jOHeFSX8iOrCIOZE24bOp0A6TQAAAAA")
         config.set(("extractor", "tumblr", "api-key"),
                    "0cXoHfIqVzMQcc3HESZSNsVlulGxEXGDTTZCDrRrjaa0jmuTc6")