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:

Given that we have no known windows developers and any we may have are extremely likely to have Chrome, it again just doesn't really matter. It's also really unlikely they would have their default set to neither FireFox or Chrome, but sure theoretically it's possible. For that to ever be used we'd at least have to set the flag name to --default though, which is another topic/bug.


> If Chrome is installed, UI will always open in Chrome even if it's not the default browser

That's the way it worked before. The only time it wouldn't work that way was if you specified the --chrome flag, which would mean to *not use chrome* and is just a flag any windows dev is extremely unlikely to use.

> If neither Chrome nor Firefox is installed, the user will just receive an error message

This is the entire bug we're fixing. So it's not a regression but rather a bug fix. The current behavior in that case is to pop up i.e. Lantern is completely broken in all versions of IE, and it just looks horrible, not to mention being unusable (it doesn't load at all).






> 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.

OK just made the change regardless. Merging!