Allow dumping pstat.Stats file to a directory for each request.

This is inspired by django-extensions 'runprofileserver'. It allows more
after-the-fact analysis looking at the output of print_stats. It also
allows use of graphical tools to analyse the stats.
Include HTTP method in filename (GET/POST/etc.)
Merge pull request #285 from rory/dump-stats-to-file

Allow dumping pstat.Stats file to a directory for each request.