Fixed Subclassing Dict Issues (Issue #101) (#102)

* Fixed Subclassing Dict Issues (Issue #101)

Previously, when child classes were instantiated, all their nodes -- besides the top node -- remained instances of the parent (Dict) class.

Added tests appropriate for these changes

addict/addict.py

@@ -22,7 +22,7 @@ def __init__(__self, *args, **kwargs):
             __self[key] = __self._hook(val)
 
     def __setattr__(self, name, value):
-        if hasattr(Dict, name):
+        if hasattr(self.__class__, name):
             raise AttributeError("'Dict' object attribute "
                                  "'{0}' is read-only".format(name))
         else:
@@ -61,10 +61,8 @@ def _hook(cls, item):
     def __getattr__(self, item):
         return self.__getitem__(item)
 
-    def __getitem__(self, name):
-        if name not in self:
-            return Dict(__parent=self, __key=name)
-        return super(Dict, self).__getitem__(name)
+    def __missing__(self, name):
+        return self.__class__(__parent=self, __key=name)
 
     def __delattr__(self, name):
         del self[name]