Add a new file: web_proxy/message.rb

In order to reduce the size of the file web_proxy.rb,
features relative to the HTTP messages have been placed
in the file web_proxy/message.rb
Add the class WebProxy::HTTPConnectionManager

This class is intended to manage the different incoming and
outgoing connections.
The class will have mainly to close connections if the timeout
has been exceeded or if the socket has been closed on the
other side.
Plus, the class will also have to handle the creation of
connections.