add widget support for iframe resizing

if liftie is embedded in an iframe it informs its parent about the ideal
height of the iframe component

the data sent includes `height` and `resort` and allows host to resize
the iframe accordingly
add script that allows to embed liftie widgets

in order to embed resort widget in the page you need to:
    <div class="liftie-widget" data-resort="alta">
    <script src="http://liftie.info/scripts/liftie-embed.min.js" defer>