Reworked GUI, closes #8, fixes #11
 - GUI now displays all the launch sites as Toggle controls. These toggles can be pressed to set the named launch site as the loaded launch site. Only one toggle can be selected.
 - Added GUIOptionGroup and GUIToggleButton to enforce the one Toggle selected rule. Seperation of concerns is achieved with a delegate that can be set when creating the GUIToggleButton and is fired when the button is enabled.
 - Removed "Launch Site Loaded" message from result label as this is made redundant by the option group. Error messages are still displayed here
- LaunchDriver modified to return the created LaunchSite. Also added method to get all launch sites and fixed #14 with a check for 0.