Check interval is done in gtk.main()

In the current cgmail 0.3.1, the checking to see whether it is time to check for new mail is done in the same thread as gtk.main(). Responsiveness is currently kept by using time.sleep(0.01), but that has the unfortunate side effect of causing cGmail to wake up 100 times/second, which can negatively impact laptop battery life by preventing lower processor C-states, and also adds a slight processor load. My suggestion is to move this checking to a separate thread, and use time.sleep(check_interval), so as to avoid this problem entirely.