Add Adyen payment gateway extension to README

Oscaro released an integration for the Adyen payment gateway. Let's show
the world!

Thank you, @metamatik.
Add __eq__ method to Price class

This commit is the squashed, but unchanged version of #1447. It adds an
__eq__ method to prices to be able to check them for equality.
Polish for Price.__eq__ changes

PR #1447 was by a first-time committer, so I did not want to rewrite
the commits.

* Removed accidentally committed file
* Style fixes

Closes #1447.