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!
Actually, today the effect of the -chrome flag on Windows is to open Lantern in the user's default browser, which could be anything (including I.E., Safari, Opera or whatever), irrespective of whether or not the user has Chrome installed.  If you leave your change as-is, there will be two regressions in behavior:

1. If Chrome is installed, UI will always open in Chrome even if it's not the default browser
2. If neither Chrome nor Firefox is installed, the user will just receive an error message

I don't know if these actually affect any real users/developers, but it seems like such an easy thing to fix by just altering the if statement that I would just go ahead and do it.  This'll be my last comment on the thread, as everything else looks fine to me.