Add the ability to insert custom managers and providers without using setuptools

[fs]

Currently all custom managers and providers must use extension points with setuptools. It would be nice if you could just give TurboMail a class reference during the initialization.

[fs]

This is also important because else you can not test anything without having a good setuptools configuration for TurboMail which can be problematic when you install multiple versions of setuptools. Taking.

[fs]

added a tearDown method so that the tests don't mess up the others

[fs]

custom providers where not initialized correctly