Seems this object should also incapsulate weakref to opened file... (to prevent using it on another text file)

* after writing, all objects, that incapsulate offset > current (during start of write()) should become invalid.
* truncate(arg) should also make that objects invalid for offsets > arg


One bad thing: seeking to the end of file is not safe:
If someone opens logfile, last byte in that text file may belong to middle of multibyte sequence. This really may happen, since Linux (libc) writes files page-by-page (if not line-buffered), so if page boundary is inside character, this bad thing may happen. Real case - is implementing something like `tail -f` in python (to, say, monitor log files).

So, seeking to the end of file is also wrong, and should be disallowed. If so, how to append text to text files ? very upset :( :( I don't know decision of that problem.