Added support for dx and dy properties for text. 

Added support for dx and dy properties for text. A bit of plumbing was already in place so I completed that.

http://www.w3.org/TR/SVG11/text.html#TextElementDXAttribute
http://www.w3.org/TR/SVG11/text.html#TextElementDYAttribute

So now a text-tag like:
`<text font-family="&quot;Helvetica Neue&quot;, Helvetica, Arial, sans-serif" font-size="18" dy=".35em" dx="1.25em">footext</text>`

is rendered .35em offset on the y-axis and 1.25em on the x-axis.

I set out to implement the Dominant-baseline property, but I was unable to do so because that requires a very intimate relationship with the font you're currently rendering and was not feasible in my available time. Dy/Dx is the next subtle way to move text around without too much hassle.