Introducing data source for PTR record generation

[johannwagner]

This introduces a data source, which can generate a PTR record on the fly with an ip address as parameter. This fixes #93.

Example:

data "powerdns_ptr" "foo" {
    ip_address = "1.2.3.4"
}

resource "powerdns_record" "foobar" {
  zone    = "in-addr.arpa."
  name    = data.powerdns_ptr.foo.ptr_address
  type    = "PTR"
  ttl     = 300
  records = ["www.example.com."]
}