CloudEvent only uses v1

[cumason123]

Expected Behavior

http_events.CloudEvent.from_http method currently only marshalls v1 events. This should probably be more dynamic in order to support multiple event versions. Perhaps we could restore specversion detection as seen in #36 or implement some other workaround.

Actual Behavior

Line 70 in https://github.com/cloudevents/sdk-python/blob/v1.0.0-dev/cloudevents/sdk/http_events.py

        event = marshaller.NewDefaultHTTPMarshaller().FromRequest(
            v1.Event(), headers, data, data_unmarshaller)