Fix KVO NSInternalInconsistencyException

> Fatal Exception: NSInternalInconsistencyException
An instance of class UINavigationController was deallocated while key value observers were still registered with it. Current observation info: NSKeyValueObservationInfo ( NSKeyValueObservance 0x..: Observer: 0x.., Key path: navigationBar.bounds, Options: <New: YES, Old: NO, Prior: NO> Context: 0x.., Property: 0x.. )

This often happens on iOS 8 when dismissing a modal view with CSNotificationView displayed, probably because CSNotificationView (with KVO) is still retained by the animation block when navigation bar deallocates. Using a weak reference could solve this issue.

I also update the `project.pbxproj` file to Xcode default format by running `pod install` with `xcproj` installed on my local machine. CocoaPods would automatically use `xcproj`, if available, to reformat the project file when integrating client projects. This could reduce diffs on `project.pbxproj`. Ref:

* https://github.com/CocoaPods/CocoaPods/blob/master/CHANGELOG.md#0240
* https://github.com/0xced/xcproj