code cleanup: create helper function _validate_section_exists and use it
code cleanup: create helper functions _validate_mandatory_* and use them + use exceptions + ...
code cleanup: move things around
move attributes out of [Ldap] in conf (preparing for shib)
add AuthShib

need the following apache conf:

  <LocationMatch "/(upload|admin|manage)">
    Authtype shibboleth
    require valid-user
    ShibRequestSetting requireSession 1
    ShibUseHeaders On
  </LocationMatch>

  <LocationMatch "/(get|meter)">
    Authtype shibboleth
    require shibboleth
    ShibRequestSetting requireSession 0
    ShibUseHeaders On
  </LocationMatch>
code cleanup: factorize duplicated code into new ldap->findRuleMatching and ldap->isDnMatching
code cleanup: introduce "ruleMatcher" object instead of "ldap" (preparing for shib rules)
add SHIB rule for shib