[APPS-1235][PRI] Add an external REST API Sample App

:koala: :+1: 

This PR intends to add a sample App that shows how to consume external REST APIs.

### Screenshots

![external_api_get_auth](https://f.cloud.github.com/assets/5967762/1947676/3159f06c-8062-11e3-8e2e-0ae9aa5f7cd2.gif)
![external_api_get_no_auth](https://f.cloud.github.com/assets/5967762/1947678/31a2b6bc-8062-11e3-90de-5d52a0fc833e.gif)
![external_api_post](https://f.cloud.github.com/assets/5967762/1947679/31a39366-8062-11e3-9cf4-cb2999748563.gif)
![external_api_put](https://f.cloud.github.com/assets/5967762/1947677/31a22e7c-8062-11e3-99a9-f714c3c76f85.gif)



### Note

You need to first register an account on teachmyapi.com using your GitHub account.

### Reference

* [JIRA](https://zendesk.atlassian.net/browse/APPS-1235)

/cc @zendesk/quokka @maximeprades @princemaple @sandlerr