Implements a new exception class for webui : HttpErrors that uses a new template : errors.html to display multiples errors

Before we were calling the logging.getLogger function without argument. The result was
that all contexts were sharing the same logger (and every message were logged X time with
X the number of loaded context).
Now getLogger is called with the context id as argument, each context has its own logger.

WARNING : not fully tested

Now the loader context is threated like the others (it loads it's libs from a symlink, before it was loading them from lodel package)

a bit dirty & quick

They were a bug when ordering stuff contains 2 values that are equals

- a "normal" import was left in lefactory_common
- the hook triggering datasources init in dyncode was not called
- dyncode was not exposed in the loader (making hook call useless : the hook has to be called once the dyncode has been "seen" (imported) by python)

Regression may have been introduced about MONOSITE mode

concerning unittest & field validators

Fixes problems with plugins

+ uses the changes in the preload method

- found a way to make it implements the socketserver.ForkinTCPServer features reducing the global code size (and removed a lot of dirty copy & paste from cpython sources)
- deleted useless imports

- the reserved context is create and set at init time by the LodelContext.init method
- allows to use lodel libs will loading
- no special features implemented yet

- updated scripts etc
- make tests pass

Method is now disabled in MONOSITE mode

- 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

Changed the exposure specs to allow giving an alias for an object when importing using the from x import y,z form

When not found return None and do not try to run len() on None

Return an instance and not a list of len 1 anymore

In construct and check data