added color icons for IOS8

On iOS8 icons for custom activities in the `UIActivityController` will be drawn in color, with the alpha channel rendered as black. As these icons are rendered in grayscale, where only the alpha channel is used and color information disregarded, the same icons cannot be used on iOS7 and iOS8.

I have created a new color icon to be used on iOS8, and I have added a switch in NKPinterestActivity to return the correct icon for the correct OS.