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

CONTRIBUTING.md

@@ -0,0 +1,18 @@
+## Contributing
+If you would like to contribute to spotipy follow these steps:
+
+### Export the needed environment variables
+```bash
+export SPOTIPY_CLIENT_ID=client_id_here
+export SPOTIPY_CLIENT_SECRET=client_secret_here
+export SPOTIPY_CLIENT_USERNAME=client_username_here # This is actually an id not spotify display name
+export SPOTIPY_REDIRECT_URI=http://localhost/ # Make url is set in app you created to get your ID and SECRET
+```
+
+### Create virtual enevironment, install dependencies, run tests:
+```bash
+$ virtualenv --python=python3.7 env
+(env) $ pip install requirements.txt
+(env) $ python -m unittest discover -v tests
+```
+