Fix #344 - better generation of user login

Code comes from mapfishapp: mapfishapp/src/main/webapp/app/js/GEOR_util.js#L120
Login is restricted to "latin" characters (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#Special_characters_in_regular_expressions)
Merge pull request #350 from georchestra/login_generation

ldapadmin - better generation of user login - closes #344