Added alt text to make Buy Now button HTML5 validate

In relation to #1, this PR adds a populated `alt` tag to the Buy Now button so it validates as HTML5:

![form submission - w3c markup validator 2014-08-18 17-32-50](https://cloud.githubusercontent.com/assets/102661/3993739/d59fabec-2910-11e4-8edd-0e574d6348a3.png)

The default text is "Buy Now", but this can be customized by providing an alternative value for the key. E.g: in _src/main/webapp/\_resources.html_:

```xml
<resources>
  <res name="liftmodules.paypal.button-alt-text">Place order now</res>
</resources>
```