Factored out layer-stealing into a separate class `CSLayerStealingBlurView`.

Moved class extension of `CSNotificationView` into a separate file.

Introduced a protocol `CSNotificationViewBlurViewProtocol` that blur-implementations have to support.

Refs GH #37
Dropped iOS 6 support. fixes GH #40
tintColor adjustments now done in the blur implementation.
Added implementation using `UIVisualEffectView`. refs GH #39
Updated icons and added @3x versions.
Added pixelmator files of existing icons to project.
Using asset bundle instead of xcassets.
Asset bundle is now generated by CocoaPods.
Updated Podfile.