Make the abstract more clear.

Hopefully.
Merge pull request #90 from noncombatant/abstract

Make the abstract more clear.
Expand on some topics in the body.

Provide a sketch of an introduction, and raise the distinction between
"trusted" (can break the security guarantee) and "trustworthy" (is unlikely
to break the security guarantee).
Merge pull request #91 from noncombatant/powerful-features-body-edits

Expand on some topics in the body.
POWER: More work.