Implement `Touch` events for OSX

[mitchmindtree]

Seems that Touch events are yet to be implemented for OSX.

[paulrouget]

Cocoa reference: https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/EventOverview/HandlingTouchEvents/HandlingTouchEvents.html

[tomaka]

Closing for rust-windowing/winit#54