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.
We should probably have a comment stating that this is still up in the air. Downloads, in I recall correctly at TPAC, did not come to consensus.