Functions to recursively walk collections which are trees.
Documentation should use Journo formats and standards.
_.walk = walk;
postorder: function(obj, visitor, context)
preorder: function(obj, visitor, context)
walk(obj, visitor, null, context)
map: function(obj, strategy, visitor, context)
pluck: function(obj, propertyName)
pluckRec: function(obj, propertyName)
_.walk.collect = _.walk.map;