[WIP] Signing of Cloudfront Canned Policies [PUBDEL-250]

Given a Cloudfront Canned Policy create a signature using an rsa private key.

The signing process itself is analog to:

```bash
echo -n "...policy" | openssl sha1 -sign private_key.pem
```

See README.md for more information.
Slight redundancy in the interface - why not ```cloudfront.SignPolicy(privateKey, cannedPolicy)``` ?