added attachBytes to Multipart.scala

OK, so I've done a small amount of work to be able to attach a byte[] instead of just a file like so:

```scala
mailer(Envelope.from("you" `@` "work.com")
         .to("boss" `@` "work.com")
         .subject("tps report")
         .content(Multipart()
           .attachBytes("ASCII TPS Template".getBytes(), "ascii-tps-template.txt", "text/plain")
           .html("<html><body><h1>IT'S IMPORTANT</h1></body></html>")))
           .onSuccess {
             case _ => println("delivered report")
           }
```
I'm pretty new to Scala, so feel free to hack away to your liking!