add --use-iam-role parameter
Update README.textile
Update README.textile
use ec2 filtering when describing snapshots

Previous design involved the API call to return all the snapshots owned
by the AWS account.
Merge pull request #1 from hroussez/master

use ec2 filtering when describing snapshots
Merge branch 'ynuyasha-master'
Add --use-iam-role option, thanks to ynuyasha (fixes #14 on github)
Make snapshot filtering more efficient, thanks to yhuyasha