multipart emails and html body

Hey Ian,

this commit includes how i'm checking for the html body part of multipart email.

Also, have a look at this commit which adds steps which explictly name the "html" and "text" parts of the the mail...  https://github.com/bmabey/email-spec/commit/e5610dfef61f5dc8fb62418f9468c0dbb349ceee