Added Email::attach_cid() returning CID which enables to embed an attachment to html

Sorry there are few more changes in this one pull request, but every this changes are connected and can't be separate. Look at commits as in individual changes. Every these changes are directed for attaching inline images, but if some commit is insufficient or something, I can rewrite it.

If you want to try demo of this functins, I'm sending email to you with demo in few minutes.