Provide some better errors messages to the user.

If the c_overviewer module is present but will not import, provide a
different error than if the modules doesn't exist at all.

Also, added a new mechanism to force users to re-build their extension
if needed (see the top of overviewer.h)

src/overviewer.h

@@ -24,6 +24,10 @@
 #ifndef __OVERVIEWER_H_INCLUDED__
 #define __OVERVIEWER_H_INCLUDED__
 
+// increment this value if you've made a change to the c extesion
+// and want to force users to rebuild
+#define OVERVIEWER_EXTENSION_VERSION 2
+
 /* Python PIL, and numpy headers */
 #include <Python.h>
 #include <Imaging.h>