remove direct support for crop.py

- it makes the code much more complicated and makes it much harder to add
  new features
- it's not used by our current work flows
- the tool can still be used with interactive_mode
mark STDOUT and STDERR unbuffered
when an assert_screen fails, save the 20 biggest changes as screens

this makes it (hopefully) possible to create needles for screens that have
a timeout (like the bootloader or the screen saver)
Merge pull request #141 from os-autoinst/issue_535

when an assert_screen fails, save the 10 biggest changes as screens