Interactive log handler introduced.

 * now it's not problem to implement interactive tools
 * in interactive mode log record ending with '... ' is treated as no newline record
Some documentation added
Errors is no longer eaten by event loop.
Error handling and logging improved.

* now worker has ability to log errors with traceback when non caught exception occurs
* `FutureResult` objects will try to save traceback when exceptions is stored in it
* exception traceback will be written to the log if nobody caught it in the last chain item