Unify attachment api

[xhit]

Merge all attachment to only one Email method called Attach and use the struct File to set the attachment information.

Fix #33

[xhit]

This PR is completed. I will left opened for reviews until Cinco de Mayo

[jlinnosa]

What I meant by the "shim" was a simple layer to translate old API into new.
e.g.

func (email *Email) AddInline(file, name, mimeType string) *Email {
        return email.Attach(&File{FilePath: file, Name: name, MimeType: mimeType, Inline: true})
}

This way you don't have to have duplicate implementations and the old tests should pass.

[xhit]

Done.