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 ;-)
@ockham What will installation look like? For example, right now we clone this repo in https://github.com/automattic/vip-quickstart and it's ready to use. I'm assuming this will add setup steps -- I think that's fine as long as it's documented.

> Are there already any other dependencies that would make sense to pull in via composer? Like e.g. PHP_CodeSniffer (see #180)?

As long as we're adding a composer step, we might as well pull in PHP CodeSniffer as well.