Start using handlers for driving the inner logic of LWP::UserAgent.

Start using handlers for driving the inner logic of LWP::UserAgent.

This also expose all the handler phases for add-ons by user code.
The supported handler phases are:

    request_prepare
    request_send
    response_header
    repoonse_data
    response_done
    response_redirect

The logic for handling of proxies are also changed in that handlers
just need to set the 'proxy' attribute of the request object for
it to be passed on to proxies.