Nicer header to avoid having the word "reference" in double.
Lower case the class variables as per the PEP 8 convention.
Call the `ExtensionSet.patch()` method instead of redefining the logic.
Fix a misaligned statement.
Add missing sections from the docstring.
Get the `ExtensionsRegistrar.register_extensions()` function to return a single `ExtensionSet` object.
Rearrange the order of the properties.
Clean-up the `Extension.__init__()` method from the parameters not required to construct the class.
Refactor the class `ExtensionSet` towards using an `add()` method.
Call the `values()` method instead of the unnecessary `iteritems()`.
Mark where a parameter can either be a single object or a list.
Add a `Settings` class.
Implement the settings within the `Extension` class.
Implement the settings within the decorators.
Implement the settings within the `ExtensionsRegistrar` class.
Remove unused imports.
Add an utility function to check if an object is a settings class.
Add a note in the doc about the settings.
Reword.
Add missing functions from the summary.