Return the value of this datahandler when empty

refs #131

The dedup mechanism was using a cast of list to set. But sometimes, filters contains list, and list are not hashable...

refs #131

Methods are written.
They are not called by anyone and the only tests that were done is that no syntax error is raised.
( refs #131 )

- deletion of unused attributes
- copy the back_reference checks in _set_back_reference function and use it in Reference.__init__()

- add some files extensions to doxyfile (*.am *.in *.ac *.patch)
- add a documentation page on autotools usage

Now when running distclean ALL files generated by the ./bootstrap && ./configure && make are deleted.
Maybe it's not a good idea because it's not the purpose of distclean target...

Using $pkgpythondir instead of $pythondir in Makefile.am
Solves a bug that put installed files in site-packages instead of dist-packages on debian

IMPORTANT : tests are broken. See details below

A new lodel/buildconf.py.ini file created. It will be transformed to a lodel/buildconf.py file by ./configure .
This file goal is to store build time constant (for the moment the presence of pymongo).
You have to build lodel2 before being able to run the tests.

- Child classes where not cleared when Plugin.clear() was called. Now child classes implements a clear_cls() method to clear a Plugin child class
- When discovering plugins if during a is_plugin_dir() check we import two times in the same virtual module name, all non existing attributes values are taken from prvious imported file (the bug was on plugin_type when webui was imported just before dummy, then dummy was an interface)

Commit refs #122 and fixes #133

- 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)

Created groups (they can be found under the "Modules" menu -_- )
Created new pages : on LeAPI EditorialModel Plugins etc

- webui fix in cookies handling
- ram_session fix in token generation (return an str : hex representation of os.urandom(n))
- more fixes in Client session handling

No need of -d flag to specifiy directories

Created a new class LodelSession to handle session datas
Created an automatic wrapper to acces to session handler plugin function throug SessionHandlerPlugin class methods
Bugfixes done for plugins. The plugin type is store in __plugin_type___ and not in __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.