Conversion to Varnish 4.0

Here is a conversion to Varnish 4.0 VMOD format.

I've just applied the same changes Varnish made to their own vmods, and removed `VARNISHSRC` reference in the readme.

The test pass but I haven't tried myself (I've made the conversion to try it on my Varnish v4 platform, which could take a while).

You'll probably want to create a new branch for this new version or for the v3 before merging: as V4 VMODs aren't compatible with Varnish 3, it's necessary to maintain both versions, but backports are easy (most changes can be merged as is in both versions).