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 a `<circle>` after mounting (such as http://jsfiddle.net/spicyj/hxFVe/), verify that the circle appears in both Chrome and IE9.

/cc @sebmarkbage