originalEvent Pass through to EventSource events (#227)

Fix for #227 

DOM events are now passed through as 'event.originalEvent' in viewer and button events where appropriate. Effects the following events:
    Viewer: 'canvas-release', 'canvas-click', 'canvas-drag', 'canvas-scroll', 'container-enter', 'container-exit', 'container-release'
    Button: 'enter', 'exit', 'press', 'release', 'focus', 'blur', 'click'