Initial read-daemon for hwmon entries

Discoveres hwmon entries for fans, input voltage, and temperature
sensors.  Polls entries on a 1s interval and displays a message to
stdout if one of the entries have changed.
diff --git a/sensorcache.H b/sensorcache.H
new file mode 100644
index 0000000..64333d8
--- /dev/null
+++ b/sensorcache.H
@@ -0,0 +1,27 @@
+#ifndef __SENSORCACHE_H
+#define __SENSORCACHE_H
+
+#include <map>
+
+class SensorCache
+{
+    public:
+        typedef std::map<std::pair<std::string, std::string>,
+                         int> container_t;
+
+        bool update(const container_t::key_type& k,
+                    const container_t::mapped_type& v)
+        {
+            auto& i = container[k];
+            if (v == i) return false;
+            else
+            {
+                i = v;
+                return true;
+            }
+        }
+    private:
+        container_t container;
+};
+
+#endif