Split common headers into a submodule and into their own files

This is a more extensible place to put them and doesn't clutter up
header/mod.rs as much as the old scheme did.

Fixes #8
Merge pull request #20 from reem/header-submodules

Split common headers into a submodule and into their own files
Added example of a concurrent server

Simple example that demonstrates how to create a concurrent
server on top of the primitives exposed by hyper.
Merge pull request #23 from reem/concurrent-example

Added example of a concurrent server
Merge pull request #19 from reem/static-response-states

Statically track the status of a Response by using a Phantom Type
Provide Response::<Fresh>::deconstruct to deconstruct a Response.

This allows downstream frameworks by-value access to these fields,
letting them create their own Response abstractions out of hyper's
Response.