Set "this" to the Dygraph object in all callbacks

Fixes #456 

I wish I'd known this was possible years ago. Passing the Dygraph object explicitly to some but not all callbacks is ridiculous. Here's the state of the world now:

  - `drawPointCallback`: Dygraph object passed as first parameter
  - `zoomCallback`: (not passed)
  - `pointClickCallback`: (not passed)
  - `highlightCallback`: (not passed)
  - `drawHighlightPointCallback`: first parameter
  - `unhighlightCallback`: (not passed)
  - `drawCallback`: first parameter
  - `underlayCallback`: third parameter
  - `clickCallback`: (not passed)

This change will give dygraphs some consistency here for the first time.