Added dependency badge for README file;

Hi, i found your library through your blog post & HN. It's nice library.

What do you think about adding VersionEye dependency badge into README? 
This badge shows a current state of dependencies and users can check out a related metadata when they click on the badge. As the author of `sabre-http`, you can also check out which [packages](https://www.versioneye.com/php/sabre:http/references) are already using yours library.