W3C endpoints for execute

[titusfortner]

Only the JSON Wire Protocol endpoints to marionette through the remote server currently work:
/session/{session id}/execute
and
/session/{session id}/execute_async

The W3C endpoints to marionette through the remote server do not currently work:
/session/{session id}/execute/sync
and
/session/{session id}/execute/async

Marionette does support them, as they work when I run them locally, directly through Ruby bindings.