Refactoring: extract common functions

- Extract database connection healthcheck to function
- Extract check for user existence in logins database to function

Signed-off-by: Ondrej Vasko <ondrej.vaskoo@gmail.com>

Add support for GeoIP2 DB (#60)

- This commit will be tagged 1.50.0

Signed-off-by: Ondrej Vasko <ondrej.vaskoo@gmail.com>

Feat/ip whitelist (#51)

* Implement IP whitelisting feature

Signed-off-by: Ondrej Vasko <ondrej.vaskoo@gmail.com>

Feat/postfwd3 support (#46)

* Upgrade script with postfwd3 plugin interface change
* Update to postfwd3 docker
* Add new geoip database path configuration option
* Update and refactor documentation
* Always log Number of unique IPs/Countries
* Add testing scripts and postgresql docker-compose
* Save request value of plugin items (`request{client_uniq_country_login_count}`)

Move documentation into directory `docs`

This will DockerHub help to properly find README file and at the same
time GitHub will show README correctly

Update README with changes for release 1.2

Version 1.1 - docker support, local dev env, minor enhancements

Stable 1.0 release