io: Add null-plugin to simplify testing.

scikits/image/io/_plugins/null_plugin.py

@@ -0,0 +1,15 @@
+__all__ = ['imshow', '_app_show']
+
+import warnings
+
+message = '''\
+No plugin has been loaded.  Please refer to
+
+scikits.image.io.plugins()
+
+for a list of available plugins.'''
+
+def imshow(*args, **kwargs):
+    warnings.warn(RuntimeWarning(message))
+
+_app_show = imshow