Use PHP VCR for mocking HTTP requests.

:warning: Do not merge yet. Work in progress.

I have done some work regarding using PHP-VCR for tests.

I'm having problems running the tests for unknown reasons. phpseclib has problems with constants being defined multiple times when using process isolation as we do now. When removing this generated classes are included multiple times.

I am unsure whether the problem is caused by PHP-VCR, phpseclib, phpunit or a combination thereof.