Implement wrappers for condition classes

[hadley]

Fixes #586

[sebffischer]

I like it. Although this is rather minimal and does not provide a lot of functionality itself, I think condition objects are fundamental to the language, so I personally think they do deserve base classes.

I also like what you said in the issue:

OTOH maybe this could be a place where we could provide more details for people who do want to extend condition classes for their own purposes?

For completeness sake, condition_message should maybe also be included here.