Implement protocol binding for DELETE /session/:sessionId/cookie

This is missing in the current protocol implementation (protocol/cookie.js). Maybe it would be great to have to have a command like `deleteAllCookies()`.

**exerpt from JsonWire-Protocol**
```
DELETE /session/:sessionId/cookie
    Delete all cookies visible to the current page.

URL Parameters:
    :sessionId - ID of the session to route the command to.

Potential Errors:
    InvalidCookieDomain - If the cookie's domain is not visible from the current page.
    NoSuchWindow - If the currently selected window has been closed.
    UnableToSetCookie - If attempting to set a cookie on a page that does not support cookies (e.g. pages with mime-type text/plain).
```