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

scikits/image/io/_plugins/test_plugin.py

@@ -1,6 +1,12 @@
-import plugin
+def imread(fname, as_grey=False, dtype=None):
+    assert fname == 'test.png'
+    assert as_grey == True
+    assert dtype == 'i4'
 
-def save(fname, arr):
-    return fname, arr
+def imsave(fname, arr):
+    assert fname == 'test.png'
+    assert arr == [1, 2, 3]
 
-plugin.register('test', save=save)
+def imshow(arr, plugin_arg=None):
+    assert arr == [1, 2, 3]
+    assert plugin_arg == (1, 2)