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.