Automatically logout wallet after 15 minutes of inactivity

This PR add an AngularJS plugin to handle events related to mouse/keyboard/window. 
The timeout is fixed to 15 minutes, but could be configured in "settings" (should be another issue).
After 15 minutes, we show a notification with a warning message. Then wait another 10 seconds and finally signout().

Please check the texts messages and tell me if need corrections.
Could we change the message to: *Your session is about to expire due to inactivity*