beginning Internet Explorer support, line height handling

- Starting now we’ll parse line-height attribute applied to the initial
<text> node and use it as the dy attribute controlling vertical spacing
of tspans if it is an . This probably doesn’t exactly map to the CSS
equivalent in terms of spacing but it does provide a quick and vaguely
standards-oriented way of handling line spacing.
- test the width of the original text node width with getBBox() to see
if we need to run the wrapping functions at all
- juggle d3-selected text node objects with the original text node as
required