[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 as described in #24 (closed)
  • adds note in README.md
  • adds POP3 functionality

Closes #24 (closed)

mentioned in commit 14fed7d1

merged