Php parser

This is a work-in-progress towards fixing #195. Commits are still a bit messy, tests are still failing, code isn't completely cleaned up, but hopefully it can serve for some preliminary review or discussion. You have been warned ;-)
Mostly in order to not introduce another dependency (even if that dependency is a dependency manager ;-) ).

Anyway, I noticed that your comment's echo triggered a short discussion on the Slack channel with @joshbetz and @ryanmarkel agreeing that composer > git submodules. As I don't have a strong preference for submodules, I'll migrate to composer (unless you disagree, that is).