Let all dynamic examples run at 60 fps

This does a few things:

* Remove call to `canvas.update()` from `on_draw` in all examples. Use a timer instead.
* Gives `canvas.update()` an optional event arg (which is not used) so it can be used as an event handler, simplifying code that wants to call `update` via a timer.
* Improve measure_fps.

Todo:

* [x] If everyone agrees with the update of the `canvas.update()` method, use it in places where we now use awkward lambda. Otherwise change where I used it already.
* [x] Update use of timer to move `connect` and `start` in the initialize call.

Closes #229
We could make the default `callback='print'` (str here is more explicit than `None`), or maybe better `callback='%s fps'` and if in the docstring say that if it's a string, it will print w/modulo the FPS. WDYT?