concerning unittest & field validators

Fixes problems with plugins

- updated scripts etc
- make tests pass

- Replaced some "hidden" import with LodelContext.expose_modules calls
 - in some functions of Setting, Plugin, validator
 - in loader.py (temporary version)

In this state the tests runs (151 success), I'm able to run an instance (from debian package using slim) in MONOSITE mode and in MULTISITE mode by creating and loading a context in the loader.

SLIM is now able to edit configuration from CLI, to start instances etc.

Now slim is able to create an instance, run make all, and start an interactive python without errors
refs #145

Able to :
- create/delete instances
- list instances
- run make %target% in all instances
- edit configuration given an instance name

refs #145

- Allow to extend easily lodel_admin.py script.
- Writtent first lodel_admin.py action : discover-plugins

Check that an EmClass.EmField exsists in dyncode

- webui now calls a hook to indicate a session end
- loader.py now set a lodel.dyncode module exposing dynmically generated code

- Add an update feature to create_instance.sh script
- Updated webui plugin load instruction (running loader.start() in run)
- Updated the way EmCmponents handles EmGroup
- Updated the way EmFields handles EmClass
- Updated xmlfile translator according to EmComponent modification

Avoid useless impossible import by putting confpsec in a dedicated file

Now it support -v and -f options (kind of dirty support for the moment...)

- 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

They have to implement a _activate() method in their __init__.py file

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

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

see README.md for more informations about instance creation