Implemented signing without encryption

Hey there,

I came across your gem, but needed the ability to sign an email without encrypting it. So I implemented this functionality. 

Just pass the "sign" or "sign_as" option without the "encrypt" option or with the "encrypt" option set to false.

Tests are included. Hope you like it and merge it into the official version.
If you have any questions or concerns, feel free to let me know.