Improve a lot error reporting and unicode/utf-8 compatibility.

Conversion between unicode and utf-8 is now done when sending/receiving
over the network by the PyminDaemon.

All errors provides now a conversion to unicode.

Dispatcher CommandNotFoundError was refactored to multiple classes with
specific and more descriptive error messages.

Handlers now can have a little help string (handler_help).

Handler's help command now list subhandlers (using handler_help).

A new error was added for help command (HelpNotFoundError).

All service handlers were updated.