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.
Updated to describe new requirements.
fix bundle image source of iOS 7
Moved sample project to subdirectory. Using own pod now.
Added class method to get the images used internally for `error` and `success` style.
Merge remote-tracking branch 'upstream/master' into develop

Conflicts:
	CSNotificationView/CSNotificationView.m
Using Asset bundle, instead of xcassets.
Fixes issues with merge conflict.

Updates share feature to latest codebase from upstream.