[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)