Making config/util/structs apis available at level 2 import (x.y), implements the roots for loopbacks (recursive transformations). This still needs work, as its hard not to close an input queue as soon as the last item was read.

bonobo/constants.py

@@ -3,6 +3,7 @@ from bonobo.structs.tokens import Token
 BEGIN = Token('Begin')
 END = Token('End')
 INHERIT_INPUT = Token('InheritInput')
+LOOPBACK = Token('Loopback')
 NOT_MODIFIED = Token('NotModified')
 DEFAULT_SERVICES_FILENAME = '_services.py'
 DEFAULT_SERVICES_ATTR = 'get_services'