Set height to 100vh

I've heard from a number of users that `core-header-panel` is confusing to use because you have to set it to fill the viewport somehow. Typically that's done by setting the html and body height to 100% and setting core-header-panel to have a height of 100%.

Setting a default of 100vh lets the user get up and running with `core-header-panel` immediately, the only gotcha is they have to remove margin from the body or it will create double scrollbars. I added a comment to the docs which points them to the [fullbleed attribute](http://www.polymer-project.org/docs/polymer/layout-attrs.html#full-bleed-ltbody).

cc @arthurevans