Support for custom thumbnails

@ebidel @addyosmani @robdodson 

- Adds a `thumbnail` attribute that can be set to the URL of a custom thumbnail image to use.
- Added another couple of tests to cover custom thumbnail behavior.
- Hopefully cleans up some element sizing inconsistency, in that the YouTube `<iframe>` and the thumbnail `<img>` (if present) are styled with `width: 100%`/`height: 100%`, and the element's `height` and `width` attributes are used to size the parent container.

Closes #9 
Thanks for pointing that out. What are your thoughts on a container `<div>` with inline `height`/`width` styles? That seems less heavyweight than pulling in `<core-style>`.

Similar to what's going on at https://github.com/GoogleWebComponents/google-chart/blob/v0.0.2/google-chart.html#L47