Improve Ldap-Sync by allowing to configure
* multiple fields which will be tried in sequence (first hit wins)
* static fields ("field"name starting with underscore, which is prohibited as
  ldap-attributeName)
* a combination of both (obviously static fields will always hit)
add documetation to AuthSyncModule::LDAP::UserSyncMap
Merge branch 'improve-ldap-sync' of https://github.com/prauscher/otrs
Added author.