Add sensor state

Add another associative container of hwmon sensor instances that
maps assorted data required to maintain the state of dbus objects.

Change-Id: I12dc7eda88e49942d98216fd06557bba87c9431c
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/mainloop.cpp b/mainloop.cpp
index 746e032..8c21566 100644
--- a/mainloop.cpp
+++ b/mainloop.cpp
@@ -36,7 +36,8 @@
       _shutdown(false),
       _path(path),
       _prefix(prefix),
-      _root(root)
+      _root(root),
+      state()
 {
     if (_path.back() == '/')
     {