CDAP-538: Ported procedures of the Purchase app to services.

@ParthGandhi The Purchase app has been updated with services instead of procedures, please take a look.
I know I recommended this, but think it maybe better to do:
```java
responder.sendString(HttpURLConnection.HTTP_NO_CONTENT, String.format("No purchase history found for %s", customer), Charsets.UTF_8)
```

Apologies for the mix-up. I just thought of this.