improve avhttp implementation

Signed-off-by: Jack <jack.wgm@gmail.com>
change email address on copyright notice

Signed-off-by: Jack <jack.wgm@gmail.com>
remvoe unused comment

Signed-off-by: Jack <jack.wgm@gmail.com>
add multi_download.hpp

Signed-off-by: Jack <jack.wgm@gmail.com>
move multi_download class to avhttp namespace

Signed-off-by: Jack <jack.wgm@gmail.com>