guideline button styles and white color

added the ability to use buttonStyle with long, medium, google or short
like the brand guidelines. added ability to use white button. added
retina support for google+ logos.
updated image location
documentation updates
labelSignin overwrites buttonStyle attribute
Switching over to use the svg version of the G+ logo.
Adding support for the `width` and `theme` attributes.

Removing the `buttonStyle` and `buttonColor`.

Also stubbed the attribute for the `height` attribute.
Updating the styling to work with the `short` and `tall` heights.
Updating the border to be consistent size.
Merge pull request #35 from Zoramite/restyling

Switching to use SVG and support official attributes for styling.
Bump Polymer to 0.3.5 & core-signals to latest release.