User-specified color

[mbleigh]

Since the spinners rely on various properties and elements for their color, I suggest adding the ability to specify the color. There are two potential ways to do this:

1. Have it as an attribute. This is serviceable, but ideally you'd be able to...
2. Have it specified via CSS. If the color attribute could specify the spinner's color that would be awesome! As it turns out, this is easier than you'd think in Polymer. See this ele for an example.