Folder fallback for configuration template file

Hi,

I would like to be able to replace standard varnish config template without changing extension code. 

I have added simple fallback mechanism which check folder specified in XML, then `app/etc/varnish` and if template does not exists in any of these use `../Nexcessnet/Turpentine/misc`.

By default module always use it's own template file, but if you need to add/change something you can simply copy standard file to `app/etc/varnish` and make changes. If you have some special deployment process and need to be able to generate template outside of magento you can specify path in XML configuration, in the `global/turpentine_varnish/config_template` node.