[#691,!395] Add Connection socket exposure and close_callback handler

[#691,!395] Add Connection socket exposure and close_callback handler

Addes close_callback and exposes Connectin's TCP socket to it
and connect_callback.

src/lib/http/client.h b/src/lib/http/client.h
    HttpClient:
        Added second parameter, socket FD, to ConnectHandler
        Added CloseHandler typedef

        asyncSendRequest() - added close_callback parameter

src/lib/http/client.cc
    Connection - added close_callback parameter to all
    methods that accept connect_callback parameter

    Added invocation of close_callback wherever the connection's
    socket is closed.

src/lib/http/tests/server_client_unittests.cc
    TEST_F(HttpClientTest, connectCloseCallbacks) - new test that
    verifies connect and close callback operations