Reorganise plugin infrastructure.

scikits/image/io/_plugins/__init__.py

@@ -0,0 +1 @@
+from plugin import *

scikits/image/io/_plugins/test_plugin.py

@@ -0,0 +1,6 @@
+import plugin
+
+def save(fname, arr):
+    return fname, arr
+
+plugin.register('test', save=save)