(feat) Set the status to 404 when no middleware handles a request.

Fixes #95
(fix) Fixed examples for 404 default.
(feat) Clone chain only once through RefCell.

Eliminates the need to do: `let mut chain = self.chain.clone()`, which is
a second copy of the entire Chain for each request. Now chain is cloned only
once per request (in the clone of IronListener) instead of twice.

This should be a performance win for complex Chains, especially those containing
large numbers of Middleware or with complicated Middleware.
(doc) Added README example and overview to crate-doc-comment.
Merge pull request #107 from reem/feat/404-on-no-handle

(feat) Set the status to 404 when no middleware handles a request.
Merge pull request #110 from reem/feat/lib-doc-comment

(doc) Added README example and overview to crate-doc-comment.
Merge pull request #108 from reem/feat/clone-chain-once

(feat) Clone chain only once through RefCell.
(feat) Added Request wrapper around HttpRequest
(doc) Documented Request wrapper.
(cleanup) Deprecated mixins.

Convenience methods can now be implemented directly on Request and Response,
with no need for additional traits to define mixins.
(feat) Added Response wrapper around HttpResponse.
(feat) Updated handle_request to use new wrappers.
(doc) Documented Response wrapper.
(chore) Added lib log as a dependency.
(doc) Documented Response wrapper fields.
(cleanup) Removed reference to mixins.
(fix) Final small fixes.