core (log): refactoring + twisted backend: - configuration is now done with classes, so inheritance is possible - twisted backend now use twisted log methods - log not produced by SàT directly in twisted backend are captured by an observer (twistedObserver) and sent back to SàT logging chain, with the "twisted" logger. - \\default output use normal twistd output, except in debug mode where it add an stdout output in addition to the log file. - when log_colors = True, colors are enabled only where it's possible (tty out), when log_colors=force colors are always enabled. - fixed some bad log.xxx calls