io: Add null-plugin to simplify testing.

scikits/image/io/__init__.py

@@ -9,6 +9,7 @@ from _plugins import available as plugins
 from _plugins import info as plugin_info
 
 # Add this plugin so that we can read images by default
+use_plugin('null')
 use_plugin('pil')
 
 from sift import *

scikits/image/io/_plugins/null_plugin.ini

@@ -0,0 +1,3 @@
+[null]
+description = Default plugin that does nothing
+provides = imshow, _app_show

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