New PHP API library

Hi all, I’ve been working on my PHP library to talk to the Discourse API and I think it’s ready for production.

This is based on a bunch of other PHP Discourse API libraries I found, though none of them seemed up to date, and were lacking many useful methods, test suites, etc.

I also added it to Packagist so you can easily install it with composer.

I am open to feedback and suggestions, but I really love pull requests. :heart: