Initial implementation

@till @dazz @ptotaram @rwos check this out...

This was a little experimental project I've been thinking about for a while. I did it now because I need something like this to cache API return payloads in easybib/api-client-php

I'm not entirely sure where this falls on the `mad genius <==> hmm <==> just madness` spectrum... it makes use of bleeding-edge metaprogramming and `eval()` to do things that are barely possible in PHP. It's based on the popular `cache_method` gem in Ruby, where doing things like this is supported much more directly by the language.

I'd love to hear your feedback.