The source/values toolkit

Me and my big mouth. No sooner do I declare the base classes "relatively
stable" than I go and mess around with it all again. Anyway, this is the
long promised set of utilities to make source/values more interesting.
It includes a few interesting little utility functions, a whole bunch of
examples and introduces the notion of "pseudo" devices with no (obvious)
hardware representation like a time-of-day device.

This necessitated making the event system a little more generic (it's
not exclusive the GPIO devices after all; no reason we can't use it on
composite devices in future) and by this point the mixins have gotten
large enough to justify their own module.

The pseudo-devices are a bit spartan and basic at the moment but I'm
sure there'll be plenty of future ideas...

docs/images/other_device_hierarchy.dot

@@ -0,0 +1,19 @@
+/* vim: set et sw=4 sts=4: */
+
+digraph classes {
+    graph [rankdir=BT];
+    node [shape=rect, style=filled, fontname=Sans, fontsize=10];
+    edge [];
+
+    /* Abstract classes */
+    node [color="#9ec6e0", fontcolor="#000000"]
+    Device;
+    InternalDevice;
+
+    /* Concrete classes */
+    node [color="#2980b9", fontcolor="#ffffff"];
+
+    InternalDevice->Device;
+    TimeOfDay->InternalDevice;
+    PingServer->InternalDevice;
+}