Now listening address, listening port and max_children count can be configured using the Settings module

concerning unittest & field validators

- 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.

Test passes

broken nginx conf + absolute path in slim that is known only at build time

- Written a new custom_list validator, it takes another validator name as argument (in order to validate items of list)
- Using it to validate list of plugins names

- The validator is written and added to all Plugin sublcasses confspec
- Adds the capabilities to give args to validator (here we give the expected type)

Import bugfixes in settings validator. Hooks must be imported in validatore definition to avoid circular import failure

Implements both Plugin child classes handling extensions and session handling.
Modified the settings plugin preload piece of code. It is now adapted to the new plugins organisation.
Each Plugin child classes provides a confspec indicating where the plugin list is stored in configuration.

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

+ 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