Add --verbose and --dry-run to GAE scraping scripts

Summary:
These scrapers are failing. These options make it easier to debug
without worrying about writing to the database. They're a precursor to
fixing the scrapers.

I updated to argparse, and then brought the three up to roughly the same
place in terms of script structure and options.

Test Plan:
I ran each script individually with the new verbose and dry-run options.
They still fail, but this makes them easier to debug.

echo test | ./dashboard_report.py -nv 0
echo test | ./instance_report.py -nv 0
echo test | ./memcache_report.py -nv 0
srcdir=. ./fetch_stats.sh -nv

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D7970