Bulking up the documentation for iron-media-query

[tjsavage]

iron-media-query is a pretty critical element but the docs currently are a bit sparse. We should try to bulk up the docs a bit and add some additional demos, with the demo code below each demo.

[Splaktar]

This is badly needed!

[ZempTime]

Hey, I'm new to Polymer, and having a lot of troubles understanding how to use this component (I'm just fiddling around in a cli-starter-kit-generated 2.x app.) I'd be happy to help PR in some documentation, especially around the the areas where I can before/after I don't get this/Aha!

A lot of my questions might be more about how how scope and state travels through web components, along with the iron-media-query specific ones. Would this be an appropriate issue to delve into those, or is there some other place I should do that?