Add support for nglviewer and fix leaflet.

[blois]

A couple of tweaks to fix nglviewer and fix leaflet:

• nglviewer expects the binary buffers to always be set (an empty array as opposed to undefined). For nglviewer does not work #13.
• nglviewer uses jQuery UI's ui.resizable extension which needs the root element to have width when initially created. Changed the root element to block styling to fix this. For nglviewer does not work #13.
• Added a placeholder shim for pWidget to improve compatibility with jupyter-widgets 4.0.0 until we get more complete support (or until 5.0 goes stable). Fix for leafmap buttons have incorrect sizing #12.