Describe plugins using .ini files, so that we don't need to import them to know their capabilities.

scikits/image/io/_plugins/matplotlib_plugin.py

@@ -1,8 +1,4 @@
-import plugin
-
 try:
-    import matplotlib.pyplot as plt
-except ImportError, e:
-    print e
-else:
-    plugin.register('matplotlib', show=plt.imshow, save=plt.imsave)
+    from matplotlib.pyplot import imshow, imsave
+except ImportError:
+    print "Could not import Matplotlib."