(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 and tests.
(doc) Small doc fixes.
Merge pull request #112 from reem/feat/error

(feat) Added real error handling through a new Error variant.
(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.
(refactor) Make status optional, defaulting to 404 on write_back.
(cleanup) Move automatic 404 generation.
(doc) Updated Request/Response documentation.
(cleanup) Switched StackChain back to default dispatch impl.