Switching to use SVG and support official attributes for styling.

Updating with a SVG of the G+ icon to scale to hidpi easily.

Adding the `width`, `height`, and `theme` attributes to match the [official button attributes](https://developers.google.com/+/web/signin/reference#sign-in_button_attributes).

The new attributes accept: 

**height**: short | standard | tall
**width**: iconOnly | standard | wide
**theme**: light | dark

An example of the updated attribute usage can be seen here: http://imgur.com/uXUTwI0