Update README.md
add zhttp spec
Update the spec to include the socket identity info

This is the spec update to go along with the http://github.com/zeromq/libzmq/pull/631 .
Adding the socket identity information to the zap requests.  This allows for
different auth selection and rules, based on different zeromq sockets.
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