Added support to manage bans and params in varnish execution module

Hi, 

I have been working in some improvements for the Varnish execution module:

* Added support for ban and ban.list
* Added support for param and param.show
* Added support to obtain complete version of varnishd (incl commit
  hash)
* Simplified purge function to use recommended Varnish 4.x syntax that
  actually works in Varnish 3.x
* Dropped support for Varnish 2.x in purge (actually it's pretty old and
  unrecommended)

Thx!