Incompatible with IE8

Raphael itself supports IE6+ actually.
But when open demo of flowchart.js on IE8, the `vml` elements were generated seen from IE inspector but not displaying at all.

sometimes, IE8 report error at line 7448 of`raphael.js` v2.1.4

```
params.font && (textpathStyle.font = params.font);
```

but since demos from raphaeljs.org works well on IE8, so I suspect some calling approach was wrong in flowchart.js