Django-oscar-adyen

- Add adyen.signers module to handle signature algorithm
- Refactor how the merchantSig is generated, using the new adyen.signers module.
- Splits constants and exceptions into their own module
- Handle shopper, billing and delivery fields (with signatures for SHA-1)
- Handle merchantSig with SHA-256 algorithm
- Improve test coverage and other minor changes

This version is backward compatible with version 0.6.0.

Note that plugin users need to implement method `get_signer_backend` if they uses their own config class from the abstract config class.

This new version (released March 1st, 2016) is mostly a "code quality" version, with small feature added to extends the plugin and to handle a new Adyen HPP field.

Also, there is a [documentation](http://django-oscar-adyen.readthedocs.org/en/latest/) now!
- Allow plugin user to extend it with get_class,
- Split several methods in order to override specific parts of the plugin,
- Expose more methods as public methods to allow plugin user to override more specific parts of the plugin,
- Add deprecation note on handle_payment_feedback and add two separates methods to handle payment return case and payment notification case.
- Add `allowedMethods` to the payment request form (unused by default).
- Start a sphinx documentation for the project.

This version is backward compatible with version 0.5.0.

Note that plugin users need to implement method `get_allowed_methods` if they uses their own config class from the abstract config class.