(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) Removed generation script.
(feat) Removed generation targets.
(feat) Added content-type dependency.

Fixes #67
Merge pull request #111 from reem/feat/generation-module

(feat) Moved code generation into its own module
(feat) Added Error variant and corresponding handlers.
(doc) Documented new Error variant.
(feat) Added chain_error to Chain and StackChain.

Fixes #106
(refactor) Refactored error-handling to work with automatic 404s.