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
I recently changed the behavior of namespaces in the reference implementation to match what's in the paper: if the unit is not in the experiment, then the default is assigned.  (see https://github.com/facebook/planout/blob/master/python/planout/namespace.py#L179)... so basically, this should instead read as:
```
$this->in_experiment = false;
$segment = ...
  ...
  $this->in_experiment = $experiment->inExperiment();
}
if (!$this->in_experiment) {
  $this->_assignDefaultExperiment();
}
```