Support validation and sending alert

The bundle file can now have a `validate` function, which when returns `false`
will fail the response.

SPAS also sends SMS alert whenever a bundle fails requesting the API, or the
response fails validation.

SMS alert uses Twilio and the credentials are defined in `config.json`:

```JSON
{
  "development": {
    "authentication": {
      "twilio": {
        "sid": "AC...",
        "authToken": "xxx",
        "number": "+18327727533", // Registered Twilio number, +1 832 SPAS 533
        "recipient": "+1xxxxxxxxxx" // Use verified outbound number associated with the number above if using trial account.
      }
    }
  }
}
```