v0.0.5.2 changes

These changes allow embedded images to show correctly in Mozilla Thunderbird (latest), iPhone Mail app (iOS 7), and Gmail web.
I have generated emails using Mozilla Thunderbird with embedded images / attachments and then reconstructed it using the Mail::Sender calls.
Please feel free to review/test (I have already tested thoroughly but you are the maintainer) and promote changes to 0.0.6