Measure memory usage in the benchmark

[jmgq]

As suggested by @pathway in #12, it would be interesting to include a memory usage report in the benchmark results. The event returned by the stopwatch in BenchmarkRunner.php allows you to get the maximum memory usage with the following method: $event->getMemory(). Unfortunately, I couldn't make it work properly, as it always returned the exact same amount, so a different approach may be needed.