Explicitly use sync versions of cookie functions

Currently, the request module does not support asynchronous cookie stores and is restricted to use only the 'sync'  cookie functions.  The `request.jar()` returns a modified `tough-cookie` jar where the async functions have been reassigned as their synchronous counterparts.  Passing on regular `tough-cookie` objects fails as request attempts to use the async functions as if they were synchronous.

In this pull request I have switched all calls to the cookie jar to reference the sync versions directly, allowing plain `tough-cookie` objects to be passed into the request object.
@lalitkapoor @mikeal To provide deeper backwards compatibility it might be worthwhile to retain a `._jar` self reference (to avoid breaking any hacks like https://github.com/mikeal/request/issues/975#issuecomment-49918100)
```js
...
   var jar = new CookieJar();
   jar._jar = jar;
   return jar;
}
```
Let me know what you think?