Add as="svg_image" and other small fixes

Hi Ray

This adds support for this awesome technique:
http://lynn.ru/examples/svg/en.html

It lets you do this:

```
{{ svgi:theme as="svg_img" src="img/logo.svg" fallback="img/logo.png" }}
```

``` html
<svg>
  <image 
    xlink:href="/assets/img/logo.svg" 
    src="/assets/img/logo.png" 
    width="100%" height="100%" />
</svg>
```

Browsers that support svg will use it, but browsers that don't will use the `src` attribute on `image` (which every browser translates to `img`)

You can then use CSS to style the size of the `svg` tag.


### Other
This PR also fixes a couple of small things:

* Fixed casing on `getCurrentthemePath`
* Use `URL::prependSiteRoot` to prevent double slashes when your theme folder is the root.