Retries For All Endpoints, fixes #347 (#457)

* test_improvements - Add __init__.py files to tests dirs so you can run all tests

* test_improvements - added helpers file, restructured tests to work without previous data and to be grouped with api type

* http_retries - Implement Retry for all requests

* Readme - Update README with contributing info

* PR Feedback - Added CONTRIBUTING.md, fixed README, fixed test

tests/helpers.py

@@ -0,0 +1,19 @@
+import base64
+import requests
+
+
+def get_spotify_playlist(spotify_object, playlist_name, username):
+    playlists = spotify_object.user_playlists(username)
+    while playlists:
+        for item in playlists['items']:
+            if item['name'] == playlist_name:
+                return item
+        playlists = spotify_object.next(playlists)
+
+
+def create_spotify_playlist(spotify_object, playlist_name, username):
+    return spotify_object.user_playlist_create(username, playlist_name)
+
+
+def get_as_base64(url):
+    return base64.b64encode(requests.get(url).content).decode("utf-8")