Problem with deallocation of custom_string instance, which was copied

Some function (insert, append , = [assign operator]) might reallocate internal char buffer. If it happens with instance, witch was copied, in copy there was stale pointer to memory area, witch was already deallocaded. This cause a problem during deallocation of copy.

Need to fix this

Possible solution:
* Add additional field, witch contains reference counter to internal buffer.
* Use smart pointer instead of a usual one