(feat) Re-export http::headers.

Just another re-export to prevent users of Iron from
using rust-http directly.
(fix) Unbox headers in Request and Response.
Merge pull request #160 from reem/feat/re-export-headers

(feat) Re-export http::headers.
Merge pull request #161 from reem/fix/rust-http-unbox-headers

(fix) Unbox headers in Request and Response.
(cleanup) Removed old response methods.

These methods are no longer applicable because Response is now
constructed by a Handler rather than modified.

Fixes #158