Non-standard async Context2d#drawImage()

If `callback(err)` is passed as the last argument to `Context2d#drawImage()`, Cairo clipping, scaling, and painting will occur within the Node thread pool.

(`err` will currently always be `undefined`, as no error checking is/was done as part of `Draw` (formerly `DrawImage`).)

`Baton` could be generalized if other functions are suitable candidates for non-standard asynchronization (I only need `drawImage` now).

This presumes Node 0.10.x+ (due to the use of domains in tests, 0.6.x+ otherwise, since I didn't bother with EIO).