Define SVG Elements/Attributes and use namespace aware javascript fcn to create.

I hope these changes are adequate to address issue #87 (SVG tags not rendered). Here I've tried to define SVG elements and attributes following the conventions you've used for HTML tags, modify driver.js to use createElementNS(), update Threepenny.hs & the .cabal file to enable building, and add a brief example. Please note/review my choice of the 'svg_' prefix to avoid name collisions (Elements, Attributes). I chose it thinking it would be straightforward for users but it feels a bit inelegant. I'm certainly open to alternatives.

Thanks for all of the energy you've given to Threepenny.