Crazy interface exmaple does not compile

Hi Basarat,
I notice that on the interfaces.md file the interface crazy could not be implemented in a class (please check here: https://github.com/Microsoft/TypeScript/issues/8917) and I made a fix with some text to explain better the pattern.
I also fixed the constructor to not return any value as it seems not to be possible. Please check here: https://github.com/Microsoft/TypeScript/issues/11588

I tried to make a pull request but it seems I got no permissions.

Start with the crazy interface and constructor examples in interfaces.md, then review the linked TypeScript issues for the reported constraints. Done means the examples can be implemented or compiled as described and the accompanying explanation accurately reflects the limitations.

Written by the indexing model from the issue text.