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
http://www.voormedia.nl/blog/2012/10/displaying-and-detecting-support-fo
r-svg-images.
Merge pull request #5 from lukereative/master

Fix SVG as an img detection for Firefox 3