Load panels with werkzeug's "import_string"

Also add back caching of the loaded panel classes.
This prevents repeated warnings if there was an import error loading the
class the first time.

setup.py

@@ -33,6 +33,7 @@ setup(
         'Flask>=0.8',
         'Blinker',
         'itsdangerous',
+        'werkzeug',
     ],
     classifiers=[
         'Development Status :: 4 - Beta',