Add Africa's talking adapter

[andela-mmakinde]

What does this PR do?

Add Africa's talking sms adapter to the application.

Description of Task to be completed? Add functionality for SMS to work with Africa's Talking. issue https://github.com/CodeForAfricaLabs/HealthTools.API/issues/56

How should this be manually tested?

• Create an account with Africa's talking, create a team and an app with them and get an API key.
• Get a shortcode users can send messages to on Africa's talking.
• Add an sms callback url <host>/sms/africastalking on Africa's talking.
• Export the variables SMS_AFRICASTALKING_USER and SMS_AFRICASTALKING_KEY.
• Start the app.
• Send a request to the shortcode registered and a response will be sent to your mobile number.

Screenshots (if appropriate)

[DavidLemayian]

What's the src/healthtools submodule for?