added user import

This adds user import (does not solve #248, but is part of it because it's required to add users which shall be assigned as contributors).
i would very much prefer the following format in this method and the method above:
```
if exit_condition
    print the warning
    return False

do_stuff
return True
```

and could you move the warning into for_each_row_in_excel_file_do? the user does not get the information about the number of columns, but i think that's okay.