Implements textAlign

resolves #5
The change looks good, you may want to sanitize this value to be only "start", "middle", or "end", with any garbage values defaulting to "start". So maybe something similar to:
```javascript
function getTextAnchor(textAlign) {
  var mapping = {"left":"start", "right":"end", "center":"middle", "start":"start", "end":"end"};
  return mapping[textAlign] || mapping.start;
}
```
This also assumes left-to-right languages only at the moment, which we can just open a different issue for.