Set previous user on all state changes if required

Fixes: #30

src/machine.rs

@@ -193,6 +193,18 @@ impl Inner {
 
     pub fn do_state_change(&mut self, new_state: MachineState) {
         let old_state = self.replace_state(new_state);
+
+        // Set "previous user" if state change warrants it
+        match old_state.state {
+            Status::InUse(ref user) => {
+                self.previous = user.clone();
+            },
+            Status::ToCheck(ref user) => {
+                self.previous = Some(user.clone());
+            },
+            _ => {},
+        }
+
         self.reset.replace(old_state);
     }