Support for forward declaration of classes.

[alexisferreyra]

• Class.$define : keeps a table of existing classes to allow forward declaration of types:

Animal = Class.$define("Animal"); // forward declaration of class Animal

Horse = Class.$define("Horse", Animal, {  /* Horse methods here */  });

Animal = Class.$define("Animal", { /* animal methods here */ });

• Class.$append : allows extending existing class object with new methods:

Animal = Class.$extend({ /* methods here */ });

Animal.$append({ newMethod: function(){ 
/*........*/ } 
});

This can be used as shortcut with $define.

• Tests fixed and tests added for new methods.