dn attribute defaults to the resource name

This allows you to do
  ldapdn{'cn=test,ou=engineering,o=mycompany':
    attributes => [ 'objectClass: top', 'objectClass: person'],
  }

without explicitly setting the dn attribute
fix problem where resources get created on each apply

On PE 3.2 (ruby 1.9.3) @work_to_do.empty? was returning false when
ensure => present with no changes
Merge pull request #11 from jonuwz/master

dn attribute defaults to the resource name