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.  This was in the s3 specific signer, but I pulled the method up into the base signer because this concept (pulling from the url) is generic for `rest-*` services as far as I can tell.

I've also added integration tests that verify we can talk to
the Elastic Transcoder service as expected.