Added possibility to respond to when the sidepanels move/pan

Added public method:
```- (void)willPanToSidePanelVisiblePercent:(CGFloat)percent duration:(CGFloat)duration```

This method gets called whenever the sidepanels moves.
Overriding this method in your JASidePanelController subclass allows you to respond to pan changes with fine grained control.
I needed this to be able to fade the background of the status bar to opaque black in iOS7 when the centerPanel gets panned to either side (Like Facebook does on iOS7)

Video for clarity: http://videobam.com/rmcUA