Adding support for jsonp request

Hi @mzabriskie 

I've used your module in my project, but sometimes I need support for jsonp request, and I find this module called [jsonp](https://github.com/webmodules/jsonp), it's very lightweight and does not have any dependencies, and it plays nicely in the browser, so I'm considering to add it to this module(as $http has this in angularjs).

I also checked the source code from [angularjs jsonpReq method](jsonpReq) but I think that is mixed up with angularjs stuff  so I don't want to use it.

I'm trying hard to follow your contributing guide as long as I can, and feel free to have me fix my code if you think there's any problem, thanks.