PlanOut PHP Port Code Review

This PR should provide the project maintainers over at [PlanOut](https://github.com/facebook/planout) an easy way to review adding the proposed PHP port contained within this repository as a submodule for the original project.

Timeline for merge:
- [ ] review proposed port in this branch
- [ ] merge changes back into this repository's master branch
- [ ] update submodule checkout commit in [PlanOut PR](https://github.com/facebook/planout/pull/31) to reflect changes
- [ ] merge port as submodule into main PlanOut repository

cc @eytan for review
Because ABLincoln is a subset of PlanOut and behaves exactly the same way as the Python version, I wonder if the README.md could be cut down substantially.  Currently it reads a little bit too much like the [original README.md](https://github.com/facebook/planout/blob/master/README.md), but replacing PlanOut for ABLincoln :).  This repeats a lot of the sales pitch that we already make on the github.io page and README, and doesn't address questions that people might have about the framework.

One idea would be to instead explain that this package is Vimeo's implementation of PlanOut for PHP and composer, and allows developers to run randomized experiments using a native, PHP-based API that is fully integrated with the standard PHP stack.  You could then give the example that you do below, and .md, and talk about how the implementation integrates with PlanOut.

In the README, you say "The implementation here serves as a basis for Vimeo's A/B Testing environment used to deploy experiments to hundreds of millions of users each month.", but this sentence sounds  a little too close to the Facebook README, "The implementation here mirrors many of the key components of Facebook's Hack-based implementation of PlanOut which is used to conduct experiments with hundreds of millions of users." Perhaps you could instead have a few sentences about how it is used at Vimeo.  One of the things that's so compelling about this implementation is that it's what you actually use in production, which speaks a lot to its stability and utility.

Finally, some users might want to know exactly how Vimeo's version differs from the Python implementation.  As far as I can tell, there are very few differences: your package includes the random assignment operators, the logging system, the experiment classes, and namespaces.  It does not include the PlanOut interpreter / language. Finally, if you tested to make sure it produces random assignments consistent with the Python version, that would be a great thing to note.  Pretty soon we will have multiple ports floating around, and it's important that they're consistent.