Added Message.attachments property
Added Message.attachments property

So user can send attachments like this:

    msg = Gmail::Message.new(
        from: "me@example.com",
        to: "you@example.com",
        subject: "Test multipart email with attachment via gmail API",
        text: "Plain content",
        html: "<p>Html <b>content</b></p>",
        attachments: {'filename.zip' => File.read('some file') }
    )

Attachments can be either `Hash` or `Array`:

    msg.attachments = ['/path/to/file', '/path/to/another/file']

glyuck/gmail-ruby-api