Server block LWRP and nxensite/nxdissite script

Created a LWRP for creating, deleting, enabling and disabling nginx server blocks. And added site enable / disable script written by @perusio
A minor style point that isn't a blocker, but I tend to favor early return vs. deeper nesting, so I'd prefer something like:

```ruby
if @current_resource.exists
  Chef::Log.info "#{@new_resource} already exists - nothing to do."
  return
end

converge_by("Create configuration file for #{@new_resource}") do
  # ...
```