add bodystructure information

hi!

i thought some information on the bodystructure might be useful, so i've created a pretty straightforward parser from what i could extract from [RFC 3501 Fetch Response](http://tools.ietf.org/html/rfc3501#section-7.4.2). The overall documentation anywhere on bodystructure sucks big time, so I tried it with about >7500 mails from the last 7 years in my gmail account, which it seemed to handle pretty ok. I tried it only with gmail, so no information about how well it handles other providers...

cheers
felix