Accept io.Reader for Attach and Embed

[arbarlow]

Here's a patch that will accept an io.Reader for both Attach and Embed with accompanying tests.

I kept the original methods named as Attach and added AttachReader though it is up for discussion!

I feel in Go the idiomatic thing would be to have

Attach(name string, r io.Reader, settings ...FileSetting)

and

AttachFile(filename string, settings ...FileSetting)

So that AttachFile is really only a convenience method. I think most people in Go would be using something io.Reader compatible?

But it works for the time being and it's sufficient enough

I wanted to add an example to the example file. But since it's pointing to a gopkg.in version that couldn't be possible at the moment without breaking the test suite.

This is talked about in issue #69

[arbarlow]

@alexcesaro what can I do to help have this merged?

[limoli]

Why are you waiting to merge this?

[pedromorgan]

see #104 (Maintainer missing) and #108