This commit removes support for the v0.2 specification. It also removes the
`contenttype` attribute from the `CloudEvent` object. While the HTTP protocol
binding specifies that in binary mode, the `datacontenttype` attribute should
map to the HTTP Content-Type header, that doesn't mean that the `CloudEvent`
object should have a `contenttype` property.
Fixes: https://github.com/cloudevents/sdk-javascript/issues/61
Fixes: https://github.com/cloudevents/sdk-javascript/issues/66
Signed-off-by: Lance Ball <lball@redhat.com>