Do not use scrot for taking screenshots

## Why?
- scrot is old (dead project)
- scrot is buggy

I'm thinking about porting [escrotum](https://github.com/Roger/escrotum) to GTK3 (and Python3), but it won't be a screenshot tool, it will be a module, that I will be able to use in _imgur-shot_. However if there was any command line tool similar to _scrot_, I'd use it instead of porting _escrotum_ (it'll take some time).
