Add a cookbook for ddtrace-php

[alexkokkinos]

The PHP Tracer does not yet have a chef cookbook similar to ddtrace-python and ddtrace-ruby. I would love to see a cookbook added eventually!

I think this would work best if the PHP tracer was published to some sort of repository. Right now my own cookbook uses remote_file and package to install/upgrade it, and must be changed manually for each release.