Added --quiet option to make aria2 quiet (no console output).
--quiet option is tagged with ADVANCED.
* src/HelpItemFactory.cc
* src/LogFactory.cc
* src/LogFactory.h
* src/MultiUrlRequestInfo.cc
* src/MultiUrlRequestInfo.h
* src/NullStatCalc.h: New class. This class prints nothing.
* src/OptionHandlerFactory.cc
* src/main.cc
* src/option_processing.cc
* src/prefs.h
* src/usage_text.h
Tatsuhiro Tsujikawa