Include percentile response times in the requests stats csv

This is the second part of PR #160:

> Include the percentile response times in the main "requests" CSV. This change could be a bit more controversial, but I don't see the reason to have to download two different CSVs containing the same set of rows, but with different data. The tricky part about this change is that it obviates the need for the percentile distribution CSV entirely, but removing it could break existing implementations. Also, there would no longer be a need both the "Max response time" and "100%" columns as these are duplicates. I left both in for backwards compatibility with any tools that analyze the CSVs directly.