Added Klein extension support.

Hello,

This pull request is to introduce support of the dynamic extension module loading into the `klein.ext` namespace.

I've also moved `_call` method from module static namespace to `Klein` object methods. This is because I'll have to inject to this exact spot in my future extension (I am planning to create an extension that introduces [`injector`](https://github.com/alecthomas/injector) support to Klein route handlers).