Fix SVG as an img detection for Firefox 3

Detection was not functioning in Firefox 3 as it does support basic SVG functions such as creating a rectangle which was the current detection method.
The new detection method was taken from this very comprehensive article - [Displaying and detecting support for SVG images](http://www.voormedia.nl/blog/2012/10/displaying-and-detecting-support-for-svg-images) - which shows that the current Modernizr method does not correctly report Firefox 3 due to reasons explained before.