New StatusBarBackgroundView Variable Mode.

This change creates 3 possible states for the status bar background view.
`MMStatusBarBackgroundViewModeNone`: The same as `showsStatusBarBackgroundView` set to NO.
`MMStatusBarBackgroundViewModeOpaque`: The same as `showsStatusBarBackgroundView` set to YES.
`MMStatusBarBackgroundViewModeVariable`: In this mode, the background view only appears when the drawer is open, with an alpha animation when closing and opening. This mimics the behaviour of the facebook app when opening the contacts drawer.

I Created 2 methods, `showsStatusBarBackgroundView` and `setShowsStatusBarBackgroundView:`, for backwards compatibility, so this change doesn't break working apps.

Also changed the example app to demonstrate this new feature.