Added note about nulls mapping to '0'
Changed vouch design to make it more secure

* See http://codesinchaos.wordpress.com/2012/09/09/curvecp-1
* Vouch is now Box [C',S](C->S')
Fixed description of INITIATE
Minor clarifications to ZAP spec & source
Merge pull request #27 from hintjens/master

Various updates to draft RFCs