Added role support

To enable role support, the aws key/secrect arguments must be omitted from
the constructors.  Otherwise, you'll get access denied.  The snapshots and sns
work for me, but I haven't tested the proxy use cases since I don't need them.
But they should work just fine.
Merge pull request #13 from shupp/master

Added role support
Tags

Tags of the volume are placed on the new snapshot

Signed-off-by: Edwin van Nuil <edwin@vannuil.com>
Merge branch 'master' of github.com:evannuil/aws-snapshot-tool

# By Bill Shupp
# Via Bill Shupp (1) and Edwin van Nuil (1)
* 'master' of github.com:evannuil/aws-snapshot-tool:
  Added role support

Signed-off-by: Edwin van Nuil <edwin@vannuil.com>

Conflicts:
	makesnapshots.py