Small bugfixes and improvements

-Fixed potential NullPointerException in Browser.openLinkInBrowser -- if the String was null, NPEs would be thrown in the catch statement
-Made emailList in Email.java consistently null (not a non-null empty list) when not using multiple email addresses
-Added simple version number to program (makes Recent Changes section more readable)
-Fixed issue with sound being able to play multiple instances at the same time (now only plays one at a time)