Support border styles

[bennyboer]

For things like horizontal rules we need to support border properties in the style file. We currently have them defined, but they are not yet used.

    BORDER("border", new StringValueParser()),
    BORDER_TOP("border-top", new StringValueParser()),
    BORDER_BOTTOM("border-bottom", new StringValueParser()),
    BORDER_LEFT("border-left", new StringValueParser()),
    BORDER_RIGHT("border-right", new StringValueParser()),
    BORDER_COLOR("border-color", new StringValueParser()),
    BORDER_WIDTH("border-width", new StringValueParser()),
    BORDER_STYLE("border-style", new StringValueParser()),
    BORDER_RADIUS("border-radius", new StringValueParser()),

[bennyboer]

Done with #11