Estended LDAP integration to allow multiple users search filter.

This is useful, for example, when your users are inside different
organizationUnit and your LDAP server is ActiveDirectory.
AD doesn't support complex search query

I've introduced new sections inside config.ini like this one

[Users:ldap:1]
# A free text name for this extra query
Alias=italy
# The organization unit where the users are
BaseDN=OU=italy,DC=insanefactory,DC=com
# The filter, which identifies the object as a user object.
SearchFilter=(&(objectClass=person)(objectClass=user))