Implements the check of the bases classes of a datasource class in a datasource plugin ( solves #144 )

refs #132

solves #58

The file is the pyconfoigure m4 macros patch that solves tyhe python3 bug and add macro to check a module version number ( refs #140 )

Now backref seems to work at creation of a Person linked with a text the Person UID is appended to Text.linked_persons

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...