io.Reader attachment support

Enables files to be easily attached to messages without requiring a disk read or manual multipart-mime creation. Makes it much easier to send in-memory bytes as file attachments.

Leverages the [simplehttp AddReadCloser()](https://github.com/mbanzon/simplehttp/blob/master/payloads.go#L72) method from the already included dependency.

Example Usage: https://gist.github.com/sym3tri/8a29ddecd65ec4f8ccfc