[core/themes] Add module-specific themes

Allow module-specific theme information to overload "default"
configuration. I.e. it is now possible to have specific prefix or
postfix configurations for different modules. The module name is derived
for each widget from the module (__module__) from which it was
instantiated.

see #23

bumblebee/engine.py

@@ -13,6 +13,7 @@ class Module(object):
     this base class.
     """
     def __init__(self, engine, widgets):
+        self.name = self.__module__.split(".")[-1]
         self._widgets = []
         if widgets:
             self._widgets = widgets if isinstance(widgets, list) else [widgets]
@@ -59,10 +60,14 @@ class Engine(object):
         """Start the event loop"""
         self._output.start()
         while self.running():
+            self._output.begin()
             for module in self._modules:
                 module.update(module.widgets())
-                self._output.draw(widgets=module.widgets(), engine=self)
+                for widget in module.widgets():
+                    widget.set_module(module)
+                    self._output.draw(widget=widget, engine=self)
             self._output.flush()
+            self._output.end()
             if self.running():
                 time.sleep(1)