Structure of successful command responses

Currently, the structure of responses for successful commands is not laid
out in the current draft of the spec. I've created a pull request[1] to try
to rectify that.

Some history: In prior versions of the spec, and in current OSS
implementations, responses have the following JSON body structure:

{ "sessionId": "<sessionId>", "status": <status>, "value":
<JSON-serialized-value> }

In Santa Clara, we decided that the first two properties (sessionId and
status) were redundant, as the information for each of those properties can
be gleaned from the HTTP response sent, The proposal in the pull request is
that response bodies for successful command executions now looks like this:

{ "value": <JSON-serialized-value> }

--Jim

[1] https://github.com/w3c/webdriver/pull/28