Improve detection of code that returns from function.

Test case missing because it needs quite complex setup and it is easier to
test it on real code.
Merge branch 'master' of github.com:yast/y2r into improve_return_detection

Conflicts:
	lib/y2r/ast/ycp.rb
Merge pull request #40 from yast/improve_return_detection

Improve detection of code that stops execution.