feat(jsonp): implement basic JSONP transport

There is a bit more that needs to be done here, and it really needs to be refactored if the connection
interface is changing, but it basically works.

Some better tests are waiting because the test suite seems to be pretty broken right now, and I am not
sure yet if Karma's webserver can be customized to make a route serve content depending on query params
(useful for JSONP), so I'm not sure how easy this is to mock yet.

Closes #9