core: new tools.sat_defer module, and implementation of RTDeferredSessions:

The real-time deferred session is a class which manage session of several deferreds at once (pretty much like a DeferredList), with the ability to get intermediate results.

src/core/constants.py

@@ -201,6 +201,7 @@ class Const(object):
     SAVEFILE_DATABASE = APP_NAME_FILE + ".db"
     IQ_SET = '/iq[@type="set"]'
     ENV_PREFIX = 'SAT_' # Prefix used for environment variables
+    IGNORE = 'ignore'
 
 
     ## ANSI escape sequences ##