Basic Twilio integration

DONE:
* Include the Twilio Ruby gem
* Added API routes to serve TwiML for voice requests
* Specs to cover basic cases during call in requests

Left TODO for SHI:
* Add logic to query and handle test results when a patient successfully enters valid username and password

To run it with [ngrok](http://ngrok.com):
```sh
~$ bundle install
~$ rails s
~$ ngrok -authtoken <your auth token> 3000
```
From your Twilio Dashboard, go to the Numbers page and set the Voice Request URL to your ngrok address with the API endpoint of `/api/call_ins/username.xml`. Then call the Twilio phone number associated with your Account.