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

src/core/constants.py

@@ -78,6 +78,7 @@ class Const(object):
     LOG_BACKEND_TWISTED = 'twisted'
     LOG_BACKEND_BASIC = 'basic'
     LOG_BASE_LOGGER = 'root'
+    LOG_TWISTED_LOGGER = 'twisted'
     LOG_OPT_PREFIX = 'log_'
     # (option_name, default_value) tuples
     LOG_OPT_COLORS = ('colors', 'true') # true for auto colors, force to have colors even if stdout is not a tty, false for no color