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!
Much closer!  The only remaining problem (which I only just recognized) is that this condition supersedes the devMode check.  I think if I'm in dev mode and specified the -chrome flag, that should take precedence over everything else.

I think the following if/else should do it (and make things a little more readable):

```java
if (LanternUtils.isDevMode() && model.getSettings().isChrome()) {
    openSystemDefaultBrowser(uri);
} else if (executable == null) {
    if (!SystemUtils.IS_OS_WINDOWS) {
        openSystemDefaultBrowser(uri);
    } else {
        // new logic here
    }
}