rewrite: alter the `send_mail` signature to take advantage of Mail

This allows you to inject a message to send or have one built for
you based on a given config block.  The block is simply eval'd on
a mail message so the API is the same as creating a mail message
manually.
rewrite: rename mailer #send_mail to #deliver

Better, more descriptive method name.  Fits more closely with Mail
and OutgoingEmail method names.  Sounds better.  I like it better.
rewrite: update spec gem description/summary

Just cleaning up and getting ready for release.