SRI: towards minimum viable SRI

This is my stab to get the spec aligned with the consensus we discussed at last week's discussions.

More commits incoming, this first one is only up to section 3.4 (excluded).
I've also filed some minor issues on the tracker (#71, #73) which are untouched by this pull request.
I wasn't sure about downloads and thought this just needs more discussion. I'm also OK with leaving it out if everyone agrees.