Slight improvements to doctest skipping.

Add a proper +SKIP directive instead of a None docstring.  While this still
doesn't report the doctests as skipped via nose, it is the correct way to do
doctest skipping, and perhaps later we can fix the nose integration so they get
correctly reported as being skipped.

IPython/testing/plugin/ipdoctest.py

@@ -186,13 +186,19 b' def is_extension_module(filename):'
     return os.path.splitext(filename)[1].lower() in ('.so','.pyd')
 
 
-class nodoc(object):
+class DocTestSkip(object):
+    """Object wrapper for doctests to be skipped."""
+    
+    ds_skip = """Doctest to skip.
+    >>> 1 #doctest: +SKIP
+    """
+
     def __init__(self,obj):
         self.obj = obj
 
     def __getattribute__(self,key):
         if key == '__doc__':
-            return None
+            return DocTestSkip.ds_skip
         else:
             return getattr(object.__getattribute__(self,'obj'),key)
 
@@ -236,7 +242,7 b' class DocTestFinder(doctest.DocTestFinder):'
 
         if hasattr(obj,"skip_doctest"):
             #print 'SKIPPING DOCTEST FOR:',obj  # dbg
-            obj = nodoc(obj)
+            obj = DocTestSkip(obj)
 
         doctest.DocTestFinder._find(self,tests, obj, name, module,
                                     source_lines, globs, seen)