DOC: Warn the user when they pass `default` to a TraitType.

Most TraitType subclasses accept a `default_value` parameter.  On more
occasions than I care to admit, I've written something like:

```
class Foo(HasTraits):
    foo_int = Integer(default=5)
```

when what I actually meant was

```
class Foo(HasTraits):
    foo_int = Integer(default_value=5)
```

I suspect I'm not the only one who's had to track down this same bug
multiple times.

IPython/utils/traitlets.py

@@ -51,6 +51,7 b' try:'
     ClassTypes = (ClassType, type)
 except:
     ClassTypes = (type,)
+from warnings import warn
 
 from .importstring import import_item
 from IPython.utils import py3compat
@@ -330,6 +331,13 b' class TraitType(object):'
         if allow_none is not None:
             self.allow_none = allow_none
 
+        if 'default' in metadata:
+            # Warn the user that they probably meant default_value.
+            warn(
+                "Parameter 'default' passed to TraitType. "
+                "Did you mean 'default_value'?"
+            )
+
         if len(metadata) > 0:
             if len(self.metadata) > 0:
                 self._metadata = self.metadata.copy()