Support better port input validation (ie. functions)

[bmeck]

related : nodejitsu/jitsu#18

Basically you have to jump through hoops right now if there is a list of properties being added to an object and you need to use a function (in this case a http request) to test if something is valid. In this case we should allow validator to be a simple function that matches :

function myValidator ( newValue, next ) {
  next( isValid( newValue ) );
}