Implemented verifying SNS bounce messages.

The following has been implemented:

1. Decoding the signature
2. Downloading and extracting the public key for the signing
certificate.
3. Generating the message to sign.
4. Verifying the message using the public key.

Some issues remain:

1. Retrieving the signing certificate every time we recieve a bounce
message is rather dumb. This is hard though because we can't just cache
a certificate without raising security issues.
2. Retrieving the signing certificate is done from any domain. This
could be probematic because anyone could craft a bounce message and sign
it using their own certificate. Amazon's documentation doesn't mention
anything about trusted domains.
3. We should do better error handling surrounding the verification
process. We should be able to handle errors loading the certificate or
verifying the signature.