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 the current Lantern, if someone running OS X or Linux only has Firefox but not Chrome, we already open the Lantern UI in Firefox.  With your change, we'll regress and instead show a message about needing to download Chrome.