Allow an array of IP ranges

One can now specify a pool range as either a single string or an array
of string. An array simply inserts multiple range declarations in the
subnet conf file. Fixes #33
Untested, something like:

```ruby
template ... do
  ...
  helpers do
    def ranges(resource_ranges)
      if resource_ranges.is_a? Array
        resource_ranges.map{ |range| "range #{range};" }.join("\n")
      else
        "range #{range};"
      end
    end
  end
  ...
end
```