Add a new_batch_http_request() method to services

This method can be used to get a BatchHttpRequest object with the
batch_uri calculated from the rootUrl and batchPath fields from the
discovery doc.

Without this change, users have to know what the batch url for newer api
endpoints is. For example:
batch = BatchHttpRequest(batch_uri=http://zoo.googleapis.com/batch)

With this change, the syntax is easier:
zoo = build('zoo', 'v1', http=http)
batch = zoo.new_batch_http_request()