Add the ability to customise the animation duration

First of all, thank you for this great work.

I just started using CWStatusBarNotification for my team's app and we needed to have faster animations so we made this little change to support it. I think it'd be good for any developer using your library to have this `notificationAnimationDuration` property (set to 0.25 by default, as it was in your define), so I make this pull request.

I updated the example to show it working too.

Thank you!