Triggered a bug in wrong Integer DH tests

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 )

Conflicts:
	examples/em_test.pickle
	plugins/webui/run.py

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

Now their is 2 macros :
 - PC_PYTHON_CHECK_MODULE_MATCH_VERSION(LIBRARY, VERSION, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND)
 - PC_PYTHON_CHECK_MODULE_VERSION(LIBRARY, MIN-VERSION, MAX-VERSION, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND)

- 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...

Conflicts:
	lodel/leapi/datahandlers/base_classes.py

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.

- now runtest runs also make refresh_plugins in the test instance

- 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