[core/output] Add widget drawing

Add basic drawing of widgets. Each module instance returns a list of
widgets using the widgets() method which is then forwarded to the draw()
method of the configured output.

see #23

tests/util.py

@@ -0,0 +1,10 @@
+# pylint: disable=C0103,C0111
+
+class MockWidget(object):
+    def __init__(self, text):
+        self._text = text
+
+    def text(self):
+        return self._text
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4