Message Signature Validation

[Pivotal Issue 78632442][1]

[1]: https://www.pivotaltracker.com/n/projects/1163646/stories/78632442
no, not as a trait, as an interface...

```php
interface CertificateProvider
{
    public function getKeys();
    public function getCertificate();
    public function getCertificateFile();
}
```

Standard implementation:
```php
class SAML2_Configuration_IdentityProvider implements CertificateProvider
{
    public function getKeys()
    {
        // omitting sanity checks for brevity ;)
        return $this->data['keys']
    }

    public function getCertificate()
    {
        return $this->data['certificate']
    }
   //etc
}
```

Our supplied implementation:
```php
/**
 * @ORM/Entity
 *
 */
class IdentityProvider implements CertificateProvider
{
    private $keys;
    private $certificate;
    private $certificateFile;

    // the getters as per interface returning the property values
}
```

> Or you will have lots of interfaces.
So far we only have this one...