heater: Add PrinterHeaters class that stores all sensors and heaters

Add a PrinterHeaters class that can stores references to available
temperature sensors and stores references to instantiated heaters.

Add a extras/heater_bed.py file and delay instantiation of the
heater_bed object.  This allows the heater.py module to be imported
earlier during the setup phase, and allows the PrinterHeaters class to
be available for registering sensors and heaters.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/heater_fan.py

@@ -3,7 +3,7 @@
 # Copyright (C) 2016-2018  Kevin O'Connor <kevin@koconnor.net>
 #
 # This file may be distributed under the terms of the GNU GPLv3 license.
-import fan, extruder
+import fan
 
 PIN_MIN_TIME = 0.100
 
@@ -20,8 +20,8 @@ class PrinterHeaterFan:
         self.fan.mcu_fan.setup_start_value(0., max_power)
     def printer_state(self, state):
         if state == 'ready':
-            self.heater = extruder.get_printer_heater(
-                self.printer, self.heater_name)
+            pheater = self.printer.lookup_object('heater')
+            self.heater = pheater.lookup_heater(self.heater_name)
             reactor = self.printer.get_reactor()
             reactor.register_timer(self.callback, reactor.NOW)
     def callback(self, eventtime):