textAlign

This pull-request introduces support for `textAlign` attribute on Text. It can be set via `attr` and allows one of the following values: "left", "center", "right". Default is "left". Partially addresses #84.

```
new Text('my Text').attr('textAlign', 'left');
```

**Why not "start" and "end"?**

It makes sense to implement "start" and "end" as soon as Bonsai also supports text direction (e.g. from right to left) similar to the [SVG](http://www.w3.org/TR/SVG/text.html#TextAnchorProperty) or [Canvas2d](http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-textalign) spec for example.