Jupyter-rfb

* Improved event specification (docs).
* Reduce throttle timeout for wheel events.
* Prevent the widget from grabbing wheel events if it just passes by during a scroll.
* The wheel event magnitude is tweaked for better consistency.

This release focuses on performance improvements:

* The widget only performs draws if it has views that are visible in the browser. So if an animated widget scrolls out of the viewport, it won't consume CPU.
* The widget now uses JPEG encoding for the images, which is much faster to encode, and faster to send over (smaller payload). There is also a new `quality` trait to control the quality. At the end of an interaction the widget always sends a lossless PNG so the static image always looks perfect.

Further:

* Improvements to docs.
* Improvements to contributor guide.

* Prevent errors raised in `.get_frame()` and `.handle_event()` from being hidden by displaying them in an outputwidget.
* Provide `.print()` to show messages in the outputwidget.
* A `.snapshot()` function to create image snapshot of the current state of the widget.
* Initial snapshots are automatically made, making off-line notebooks more useful for docs and tutorials.

* Improvements to the docs, dev process, binder support.
* The default size was changed (width changed from "100%" to "500px").
* Minor tweak to when the stats start measuring.

First release.