Remove unused import

Signed-off-by: Michal Čihař <michal@cihar.com>
Explicit notion of abstract classes

Signed-off-by: Michal Čihař <michal@cihar.com>
Move functions out of class

Signed-off-by: Michal Čihař <michal@cihar.com>
Make it clear this class is abstract

Signed-off-by: Michal Čihař <michal@cihar.com>