bug 840240 - Remember user 's preferred view of crash-stats

This makes it so that whenever you click on "By Build Date" instead of "By Crash Date" it remembers that and pre-selects it for you next time. 

This choice is also carried with you as you go from the home page to the Top Crashers page. And equally if you change your mind in Top Crashers and return to the home page, it's remembered there too.