[trees] - install dovecot from debian backports, enable pop3

  * adds compatibility for dovecots maildir
  * adds role for installing debian backports
  * installs newer version of dovecot 2.2.* from debian backports
  * in newer versions of dovecot >= 2.2.31 cache index corruption
    is fixed
  * adds note in README.md
  * adds POP3 functionality