Commercetools

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- Add ability to run the mock server standalone using:
`python -m commercetools.testing.server`
- Add paginator class
- Add support for delete operations in the mock services
- Support oauth2 authentication via basic auth and querystring in the mock
service
- Properly support predicate filtering in the mock endpoints using a custom
query parser.

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- Add HTTP retry logic for status codes 502, 503, 504 (retry 3 times).
- Add Client.products.upload_image() endpoint
- Add Inventory endpoints

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- Add documentation via read the docs

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- Add support for custom objects (including mock endpoints)
- Fix handling of discriminator fields

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- Add support for channels endpoint
- Fix packaging issues
- Workaround a bug in marshmallow_enum ini combination with many=True

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This is the initial opensource release of the Commercetools Python SDK.

- Add travis support in combination with tox
- Properly support passing list and scalar values to where, sort and expand