ImportError: No module named Bucket

A --dry-run on delete fails with
        DETAIL: Traceback (most recent call last):
          File "/usr/local/lib/python2.7/dist-packages/wal_e-0.7.dev-py2.7.egg/wal_e/cmd.py", line 365, in main
            import boto.s3.Bucket
        ImportError: No module named Bucket

This fixes the import
Add Christian Pedersen to CONTRIBUTORS

Signed-off-by: Daniel Farina <daniel@heroku.com>