Add sender header

This changeset adds the capability to display the **Sender** header when specified.

This is useful, based on [this Stackoverflow answer](http://stackoverflow.com/questions/4728393/should-i-use-the-reply-to-header-when-sending-emails-as-a-service-to-others) in regards to a SaaS product that would allow a client to author an email that would be sent as part of the SaaS product.

This is my first contribution to an open source codebase, so please let me know if there is anything additional you may need to get this changeset merged into your codebase!

**ALL SPECS _PASS_**

Thank you.