Forcibly wrap SVG nodes with <svg> on creation

Forcing wrapping seems necessary here: I compared a <circle> created within a <div> with a <circle> created inside an <svg> and they appear to have exactly the same properties with the exception of .parentNode (and .parentElement), yet the former refuses to show up when appended to an <svg> element. As such, I can't find any useful way to write a unit test (testing getMarkupWrap's output doesn't seem particularly useful to me).

Fixes #311.

Test Plan:
With a component that adds an <circle> after mounting (such as http://jsfiddle.net/spicyj/hxFVe/), verify that the circle appears in both Chrome and IE9.

/cc @sebmarkbage