Add methods to the HTTPResponse class

The three added methods aim at accessing the components of
the start line more easily:

- HTTPResponse#version

- HTTPResponse#status

- HTTPResponse#reason_phrase
Add a logger as attribute of the class HTTPProxy

The logger will provide basic information about the state of the
proxy, the incoming and outgoing connections.
It will also certainly be used for future debugging purposes.
Change the method HTTPRequest#host

The methods HTTPRequest#host and HTTPRequest#request_uri may
not be able to rely on the module URI, because a bug has occurred
when the framework was used with a real web browser.
Further investigations may be needed.
Add the class WebProxy::URI

There was a bug with the standard module URI and the associated
function named URI().
In order to fix this bug, a class URI has been created.
Fix the problem relative to the version number

The version number, namely WebProxy::VERSION, was not a string.
It has been transformed into a string.