(feat) Wrapped http::Request and http::Response

!#[breaking change]

Request and Response are now iron-defined wrapper structs with ergonomic
fields and convenience methods that don't require mixins. This is a short-term
bug that will require changes but is a huge long-term win as it gives us total
control over our API and removes our direct dependency to rust-http.

Fixes #105
Fixes #94
Fixes #93
Delaying the writing allows headers to be written through successive mutations of the headers structure, even if there were calls to `serve` in between. :+1: