Add support for additional *headers* in backend REST

[Antwnis]

When Kafka REST is installed in a header protected end-point, a set of additional Headers might need to be delivered on every API request:

-H "X-Auth-Token: APIKEY"

For example on BlueMix: https://console.bluemix.net/docs/services/MessageHub/messagehub025.html#rest_using

env.js should provide mechanisms for additional headers