added possibility to sign a message (DKIM)

I have only implement the DKIM signer, but someone can easily add `\Swift_Signers_SMimeSigner` and `\Swift_Signers_DomainKeySigner` or a custom `\Swift_Signers`

```yaml
# app/config/config.yml
lexik_mailer:
    ...
    signer:               "dkim"
    dkim:
        private_key_path: "%kernel.root_dir%/config/dkim.key"
        domain:           "mydomain.com"
        selector:         "myselector"
    classes:
        signer_factory:    "Lexik\Bundle\MailerBundle\Signer\SignerFactory"
        signer_dkim:       "Lexik\Bundle\MailerBundle\Signer\DkimSigner"
```


If you want to add a signer, create a new service with the tag `lexik_mailer.signer`. The service need to implement `SignerInterface`

```xml
<service id="lexik_mailer.signer.mysigner" class="%lexik_mailer.signer_mysigner.class%">
    <tag name="lexik_mailer.signer" label="mysigner" />
</service>
```