Check that an EmClass.EmField exsists in dyncode

- Replaced a self. by cls. in leobject
- Changed default value for runtest option

- now datasources are declared with two conf keys : read_only and identifier
- the old datasources plugin hook is now declared in lodel/plugin/core_hooks.py. This file is imported after lodel/plugin/hooks.py in the loader.py

- renamed Plugins class to Plugin
- an instance represent a loaded plugin
- classmethod allows to preload & load plugins

+ a new validator named 'string', it tries to cast the conf value using str()

To start in standalone uwsgi set lodel2.webui.standalone=True in conf file and start loader.py

- fixes exceptions output
- add "none" validator that validates only None