Improves sendCookie handling

This issue will probably take some time. We are not actively working on it (since we don't really use this). So any contribution is welcome. This will address some of the issues in PR #49.

1. sendCookie should actually synchronize the cookies from `NSHTTPCookieStorage` to `document.cookie`. This will enable AJAX requests to share the cookie. You can simply use `document.cookie = newCookie;` to set/update cookies.
2. Add a new prop like `updateCookie` to force an update from `Set-Cookie` header to `NSHTTPCookieStorage`. I believe `Set-Cookie` will eventually end up in `NSHTTPCookieStorage`, it's just a matter of when.
3. Maybe also expose `webView:decidePolicyForNavigationResponse:decisionHandler:` and `WKUserScript`.