Examples stop io_service.

The example programs did not finish; by supplying and asio::io_service and invoking 'stop()' on that services the examples should finish correctly.

What to do with the documentation?
Currently I only updated the example code in the documentation.
- On one hand, users might be confused about the `client_options`, the `shared_ptr<asio::io_service>`, etc., thus I should describe their use; and
- one the other hand, `client_options` and `shared_ptr<asio::io_service>` distract from the simple proces of build-request, create client, and process response.

Not fixed:
- hello_world_client.rst and http_client.rst appear to be duplicates, perhaps to remove one of them.
- same issue for http_client.cpp and http_client1.cpp

~~ Kasper van den Berg