Static route tests and documentation. (silverstripe#179)

* Added `getStaticEndpointUrl` helper method.
Restructured `PaymentGatewayController` code, so that public methods come first.
Added unit-tests to test the new static-route feature.
* Added documentation about static-route feature.

Fix typo in documentation.

Adding changelog for 2.2

Adding 2.1 changelog

Merge branch 'master' into 2.0

Allow failure of tests against core-master