Added the logger part of the Uaapi.

* The logger has 4 levels: INFO, DEBUG, WARN, ERROR.
* It is used via the macro ulog(LEVEL) like the following:
  ulog(INFO) << Some message.";
* Via the Uaapi::Log class, user can:
** Get the minimum log level (Log::getMinLogLevel()).
** Set the minimum log level (Log::setMinLogLevel()).
** Log to a given stream or a given file, opened with the given mode
   (Log::logTo()).
** Get and set a flag that determines if the program should abort on ERROR log
   message (Log::failOnError()).
** Retrieve the stream on which the logger writes (Log::getLogStream).
Added the Scons sconscript and SConstructs files to build the Uaapi.