A few fixes for SVG features . . .

* Fixes the issue I was having with transparent SVG in [https://github.com/lucasmazza/spriteful/issues/13]
* Uses svgasimg from modernzer [https://github.com/Modernizr/Modernizr/pull/1209]
* Add an option to skip svg optimisation
* Fix IE9 display by base 64 encoding the svg.