Added sound on/off option, modified GUIs

-Added a JCheckBox to the Setup GUI to turn the alert sound on or off, made Test Play Sound button disable if sound is disabled
-Made update GUI display update size (in MB) before downloading
-Made update GUI display download percent complete
-Added "Recent Updates" tab to Setup GUI
-Added info to Extras tab
-Made ErrorManagement printStackTrace of error given
-Added a few Javadocs
-Added nullchecks to Browser.java (so it won't nullpointer if a PAXChecker GUI object is null for some reason)
-Made update GUI remove "Yes" and "No" buttons when downloading update
-Made Setup check boxes for scanning Showclix website and playing alert when update found checked by default
-Made Email.sendMessage() not try and send a message to a null email address
-Renamed some PAXChecker.java variables/methods to more consistent names