Closes #1684

This adds code to make sure the user is notified if he or she does not have chrome or firefox and needs to download one of them to run Lantern.

This goes a bit beyond the original ticket in that it adds a check for firefox and open Lantern in FireFox if it finds it. 

If you can have a look @oxtoacart that would be great!
In other words, on windows the flag doesn't make much sense, but more than anything I don't think it makes any difference either way so probably isn't worth spending time on.