added support for getting absolute url for an image when used with <asset:image> tag

I was using asset plugin in my project and needed to render a template in email which used \<asset:image\> tag.

The template created relative urls but since the image was part of the email, it needed an absolute url for google to find it. So I added an additional attribute for getting complete URL of the image.

It can be used like this

 \<asset:image src="myImage.jpg" absoluteUrl="true"/>
One other suggested change here. Note when using a CDN via the grails.assets.url config you want to replace serverURL with the assigned grails.assets.url i.e. conf.url as this will be absolute.