add upload/2 function.

upload a blob using the standalone API.

Example of usage::

    test() ->
        coffercli:start(),
        Conn = coffercli:new_connection("http://localhost:8080"),
        Storage = coffercli:storage(Conn, <<"default">>),
        Resp = coffercli:upload(Storage, <<"hello world">>),
        io:format("got ~p~n", [Resp]),

        {ok, Client2} = coffercli:upload(Storage, {stream,
                                                   <<"hashtype-20130608test10">>}),
        {ok, Client3} = coffercli:send_blob_part(Client2, <<"hello
world\n">>),
        {ok, Client4} = coffercli:send_blob_part(Client3, <<"welcome on
earth\n">>),
        Resp1 = coffercli:send_blob_part(Client4, eob),

        io:format("got ~p~n", [Resp1]),
        Resp2 = coffercli:upload(Storage, {file, "/etc/hosts"}),
        io:format("got ~p~n", [Resp2]).