added new functionality:
* `requests.cookies.get_cookie` - returns an actual cookie object
* `requests.cookies.remove_cookie_by_name` - simply adds the `remove_cookie_by_name` function proxy into the main class ( to simplify imports and the interface )

also added tests:
* `test_cookiejar_session_operations` - ensures expected cookie operations work ( existing and new )
* `test_cookiejar_session_domains` - ensures setting/deleting cookies are domain specific