Allow 'stop' elements with CSS syle attributes to be parsed

[chrisgibbs]

Allows parsing of gradient stops that use CSS styles instead of XML style attributes.

Ex:

    <linearGradient spreadMethod="pad" id="gradient" x1="0%" y1="0%" x2="80%" y2="116%">
      <stop offset="2%" style="stop-color:#09326C;stop-opacity:1;" />
      <stop offset="100%" style="stop-color:#CD519D;stop-opacity:1;" />
    </linearGradient>

where stop-color and stop-opacity are defined on the style attribute.