Decoupling SBWindow from SBBark.

I already handle a shake window for other purpose. 
So I've decouple SBBark (core piece of SBWindow) from SBWindow, which now just handle the shake motion and dispatch event, which can be observe to present the SBBark shared instance (but deny still using the delegate).

I hope it's clean and make sense.

Hugues

More info:

Decoupling SBWindow from SBBark (handling the github issue and mail feedback). Allowing to call bark by itself ([[SBBark sharedBark] showBark]) and give more flexibility to invoke Bark.

Also SBWindow become more generic (dispatch a NSNotification) and can be use by other class to handle shake event (as [SBBark sharedBark] still support a delegate to verify if it should present).