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!
Well, the funny things about the --chrome flag check are:

1. It's really more of a --default flag, as it just says to use the default browser
1. It's only really relevant on OSX, as on Windows and Linux Lantern will open for almost any developer in Chrome or Firefox anyway.

It's not really consequential either way, though, and is also going away along with all other java code.