Adds first step towards adding integration tests for existing features.

Integration tests are currently run by the private Codeship project, but can easily be run by anyone during development:

```bash
export ESENDEX_USERNAME=you@domain.com
export ESENDEX_PASSWORD=super-secret-password
export ESENDEX_ACCOUNT=EX123456
export ESENDEX_MOBILENUMBER=447000000000

gulp integrationtest
```

Use `SET` for Windows environment variables.