A script to create snapshots of a volume and delete old snapshots.

Summary:
You specify how many snapshots to keep.  By default, this keeps the
most recent n/2 daily snapshots, the most recent n/4 weekly snapshots,
and the most recent n/4 monthly snapshots.

Inspired by http://www.geekytidbits.com/rolling-snapshots-ec2/.

Add a note about setting up PATH

A script that makes a (rolling) snapshot of the data on toby.

Run a daily snapshot of toby's data disk at 4am.

Test Plan:
Ran the following and verified they gave an error:
  ./ec2-create-rolling-snapshot.py -n
  ./ec2-create-rolling-snapshot.py -n -d 'backup of toby data'
  ./ec2-create-rolling-snapshot.py -n -d 'backup of toby data' -v vol-06f30e77
  ./ec2-create-rolling-snapshot.py -n -d 'backup of toby data' -v vol-06f30e77 -m 10
  ./ec2-create-rolling-snapshot.py -n -d 'backup of toby data' -v vol-06f30e77 -m 10

Then ran the following and got this output:
  % ./ec2-create-rolling-snapshot.py -n -d 'backup of toby data' -v vol-06f30e77 -m 10 -K ~/aws/pk-backup-role-account.pem -C ~/aws/cert-backup-role-account.pem
  [DRY RUN] Created snap-TBD from vol-06f30e77

It's not a great test; we'll see more as time goes on.  I'll have it
email me every day until we get to see it actually start deleting
things (in like a year's time!, since we're allowed 500 snapshots)

Reviewers: benkomalo, alpert

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2006