Add sendJsonp and map to Http module

Open to suggestions. In particular, not sure about Http.map (what happened to #411?). Also not sure if we want to roll JSONP requests into Request, or have our own timeouts (JSONP can't return Failure right now, just an empty Success or stuck on Waiting). A bit of code duplication in Http.js too.

Example of using JSONP for Wikipedia API lookups:
https://gist.github.com/osnr/10723233
(hosted at http://rsnous.com/demos/elm-jsonp/Jsonp.html)