Changes to @dcu's code that I'd absolutely need

Hi, I was looking at your `master` and thought I'd share how I had to work around some of @dcu's code that you have also merged.

It does one rather violent thing: whenever looking up any info at all about a Message, it'll fetch its' content, marking it as read in Gmail.

I posted this note in https://github.com/nu7hatch/gmail/pull/66, and wouldn't expect it to be merged in to the gem as is because of this, sadly.

This PR is not for merging, sorry about the format, it's just to illustrate the point. 

I picked up the history from my own manual merging of @dcu's fork, so there's a bit of noise pertaining to the IMAP extensions fixes. The important part is the difference in `message.rb` and the calls to `#fetch_email_data`. 

Basically mine doesn't use `RFC822`, because that's what causes the whole email to be read, so here you'd have to call `#message` to load it, which is more in line with the gem's expected behaviour.

Cheers