Give subclasses of the Instance traitlet the option of overriding the klass variable to have a default klass.

This makes it easy to construct traitlets for specific object types and default metadata.  For example, we can have a traitlet for numpy arrays that gives default values for `to_json` and `from_json`:

```python
def np_to_list(a):
    if a is not None:
        return a.tolist()
def np_from_list(a):
    if a is not None:
        return np.array(a)

class NDArray(Instance):
    klass = np.ndarray
    metadata = {'to_json': np_to_list,
                'from_json': np_from_list}
```

Then the NDArray traitlet will by default have the klass and some custom metadata.

IPython/utils/traitlets.py

@@ -809,8 +809,12 b' class Instance(ClassBasedTraitType):'
     """A trait whose value must be an instance of a specified class.
 
     The value can also be an instance of a subclass of the specified class.
+
+    Subclasses can declare default classes by overriding the klass attribute
     """
 
+    klass = None
+
     def __init__(self, klass=None, args=None, kw=None,
                  allow_none=True, **metadata ):
         """Construct an Instance trait.
@@ -836,14 +840,17 b' class Instance(ClassBasedTraitType):'
         -----
         If both ``args`` and ``kw`` are None, then the default value is None.
         If ``args`` is a tuple and ``kw`` is a dict, then the default is
-        created as ``klass(*args, **kw)``.  If either ``args`` or ``kw`` is
-        not (but not both), None is replace by ``()`` or ``{}``.
+        created as ``klass(*args, **kw)``.  If exactly one of ``args`` or ``kw`` is
+        None, the None is replaced by ``()`` or ``{}``, respectively.
         """
-
-        if (klass is None) or (not (inspect.isclass(klass) or isinstance(klass, py3compat.string_types))):
-            raise TraitError('The klass argument must be a class'
-                                ' you gave: %r' % klass)
-        self.klass = klass
+        if klass is None:
+            klass = self.klass
+        
+        if (klass is not None) and (inspect.isclass(klass) or isinstance(klass, py3compat.string_types)):
+            self.klass = klass
+        else:
+            raise TraitError('The klass attribute must be a class'
+                                ' not: %r' % klass)
 
         # self.klass is a class, so handle default_value
         if args is None and kw is None: