Fix label position in IE SVG renderer

Label position in OL SVG renderer is set using the 'baseline-shift' property but this property does not work in IE as can be check here: 
http://www.w3.org/Graphics/SVG/Test/20061213/htmlObjectHarness/full-text-align-02-b.html
As a workaround, the 'dy' property can be used to get a similar result. Explained here: http://nelsonslog.wordpress.com/2011/09/12/svgtext-baseline-considered-harmful/