adding logs when adding a user to a group

adding exception to prevent using skeleton module

fix backend module skeleton

fix unit test and fix the way ldap connexion options are handled

fix add/del groups

fix API for backend ldap on groups handling

multiple modification in backend ldap ...

* better handling of which attributes is recovered
* fix set_attrs method
* change api of set_attrs method for constancy
* change calls to _get_user and _search methods (internal api change)

implementing modify and add/rm to groups

removing trailling spaces

adding key handling

reorganize the ldap backend module

correct backend module skeleton

add exception handling

implement get_groups

fix logging in ldap backend

reimplement get_user in ldap backend

adding methods add_user and del_user ...

* adding add_user
* adding del_user
* adding unit tests
* adding configuration parameters for adding users

adding objectclasses options

factoring binding to the ldap

code factoring

implementing search users ...

* adding search
* adding unit tests

adding search template

adding skeleton of method search user

fix method arguments

more logical return for get_user

multiple changes ...

* implemeting recover user attributes
* adding a unit test for unavailable ldap
* adding a parameter timeout to set the ldap timeout connexion

add passing attributes list to backend

adding the possibility to get a list of attributes in get_user

adding close connexion and exception handling

fix wrong call to method