Rewrite Avoding Key Reuse section

* Near-complete rewrite focusing on brevity.

* Remove security arguments entirely. The financial privacy arguments
  are strong enough to make the case on their own for avoiding key
  reuse. The security benefits are a (now-unmentioned) bonus.
Revise new Avoiding Key Reuse section based on feedback from @saivann
Merge pull request #55 from harding/devkeyreuse

Rewrite Avoding Key Reuse section