Grammar did not define initiate-cookie
Added explanations of vouch and nonces
Fixed link to reference implementation
Minor fixes
Changed language to use 'permanent' and 'transient' for keys
Clarified that identity frame cannot be multipart
Removed reference to old command formatting
Spec wrongly said NULL mechanism was 5 chars
Minor clarifications
Merge pull request #26 from hintjens/master

Changes to RFCs as part of stabilising ZMQ/4.0