Fix for #338 (clear reader on signout)

App now sends a broadcast upon signout which is received by WPActionBarActivity.

WPActionBarActivity calls onSignout() when this broadcast is received, so descendants can do activity-specific cleanup when the user signs out.

NativeReaderActivity relies on this to clear the post fragment when the user signs out (otherwise, the previously displayed posts will continue to show if the user logs back in)