New module: rax_dns_ptr

This module is designed for creating reverse DNS records (PTR records).

Sample play:

```
---
- hosts: localhost
  gather_facts: False
  tasks:
    - name: Create server
      local_action:
        module: rax
        credentials: ~/.raxpub
        name: ptr.example.org
        flavor: performance1-1
        image: ubuntu-1204-lts-precise-pangolin
        disk_config: manual
        region: ORD
        state: present
        wait: yes
        wait_timeout: 360
      register: rax

    - name: Create PTR record
      local_action:
        module: rax_dns_ptr
        credentials: ~/.raxpub
        name: ptr.example.org
        server: "{{ (rax.instances|first).rax_id }}"
        address: "{{ (rax.instances|first).rax_accessipv4 }}"
        region: ORD
        ttl: 3600
      when: rax.action != 'delete'

    - name: Modify PTR record
      local_action:
        module: rax_dns_ptr
        credentials: ~/.raxpub
        name: ptr.example.org
        server: "{{ (rax.instances|first).rax_id }}"
        address: "{{ (rax.instances|first).rax_accessipv4 }}"
        region: ORD
        ttl: 900
      when: rax.action != 'delete'

    - name: Delete PTR record
      local_action:
        module: rax_dns_ptr
        credentials: ~/.raxpub
        name: ptr.example.org
        server: "{{ (rax.instances|first).rax_id }}"
        address: "{{ (rax.instances|first).rax_accessipv4 }}"
        region: ORD
        state: absent
```