Decrypt secure:: parameters

If a config file is specified, this searches for parameter values prefixed with `secure::` and decrypts them using a private key file that must be specified in `~/.cfnrc`. [ursa](https://github.com/Obvious/ursa) is the node library used to run the decryption, which wraps `openssl`.

Encrypting content to put into your config file looks like this:

```sh
echo -n 'super duper secret' | openssl rsautl -encrypt -inkey ~/.ssh/public.pem -pubin -oaep | openssl enc -base64
```

Then .cfnrc needs to look like this:
```json
{
    "accessKeyId": "xxxx",
    "secretAccessKey": "xxxx",
    "secureKey": "/full/path/to/private.pem"
}
```

refs #4 /cc @willwhite @ianshward