Patch 1 (changes to handle canvases and events in iframes)

I made changes to CanvasView and View in order to make paper with canvases located in iframes.

The test (canvas instanceof HTMLCanvasElement) in CanvasView fails when the canvas is in an iframe. I loosened the test though there may be better approaches.

In View document event handlers are added when the canvas.ownerDocument is not document. They are removed when there are no more views referencing the canvas.ownerDocument.

I apologize patch name and for the edit to master - this is the first time I have used github.

I'm interested in any feedback on my changes and misses - I'm not an expert this environment.