Adding composer.json

Hello,
Great work on this wrapper ! 

I use it with a framework relying on http://getcomposer.org 
With the composer.json file in the repository, I'm able to manage directly phpZenfolio library, and get the needed version from github.

(It's for me the first time that I made such a composer.json file, so there could be beter way of doing it. But it think it's not too bad...)

Thanks for considering this pull request.