[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.
Why not use http://golang.org/pkg/bytes/#Map ?