Fix elastictranscoder service

This fixes two issues to get elastictranscoder working:

* account for the `rest-json` service type in `get_response()`
* account for query strings coming from the url path in sigv4 signer

I've also added integration tests that verify we can talk to
the Elastic Transcoder service as expected.
Merge branch 'rest-json' into develop

* rest-json:
  Fix elastictranscoder service