Change the Handler trait to receive an Iterator of (Request, Response) pairs.

This allows downstream users to have total control of their concurrency
strategy, while also exposing a very nice, streaming interface for frameworks
to build on.

This also resolves issues surrounding the use of IoResult as the return type
of Handler::handle, because handlers now have complete control over how to
handle internal failure.

Fixes #3
Fixes #4
Updated example for new Handler trait

This introduces a bit of complexity to the example,
mainly the use of the try_continue! macro for dealing
with errors, but I think this is an appropriate trade-off
as users of this library are likely to be framework authors
instead of end users.
Merge pull request #13 from reem/handler-iterator

Change the Handler trait to receive an Iterator of (Request, Response) pairs.