Reinstate rediscovered material

e1d5c28 removed a paper because of a broken link, this restores
it with a new link.
Mention a second model tool: concuerror
Add 'WhatsApp' to the list of companies using Erlang

It isn't secret in the least; it's mentioned on their blog:
  http://blog.whatsapp.com/2011/09/?lang=en
and at least one conference has had a presentation about it.