Add support for progress events. ~34

[marshallswain]

I'd like to add progress event support to be able to test a file upload module I'm making. I don't have a lot of time for it, so maybe @justinbmeyer can help with a fixed-on-paper or we can pair sometime?

I found another module that implements this, so maybe it will help: https://github.com/philikon/MockHttpRequest/blob/master/lib/mock.js#L325

[justinbmeyer]

@marshallswain can we talk about this tomorrow? I might be able to squeeze it into the 0.2.0 release.