Implemented persistent size/location on dialogs. 

Implemented persistent size and location of all windows *except* the
main window via a file on the user's computer. (Issue #13)
Renamed old TriviaDialog class to the more accurate TriviaDialogPanel.
This also opened up namespace for a TriviaDialog that actually extends
JDialog.
Moved all of the recurring code for dialog boxes (including the internal
JOptionPane) into the custom TriviaDialog class.
Implemented persistent size/location for main window.

Moved saveLocation and loadLocation from TriviaDialog into static
methods in TriviaClient.
Added persistent size/location to the main client frame.