New module: nexmo

This pull request adds a new notification module for sending notifications/messages via the nexmo API.

Example usage:

```
- name: Send notification message via Nexmo
  local_action:
    module: nexmo
    api_key: 640c8a53
    api_secret: 0ce239a6
    src: 12345678901
    dest:
      - 10987654321
      - 16789012345
    msg: "{{ inventory_hostname }} completed"
```