update version to v2.8.1

Signed-off-by: Jack <jack.wgm@gmail.com>
add LICENSE for avhttp.

Signed-off-by: Jack <jack.wgm@gmail.com>
change BOOST_ASIO_DECL to AVHTTP_DECL for multi_download.hpp

Signed-off-by: Jack <jack.wgm@gmail.com>
fix error_code no clean.

Signed-off-by: Jack <jack.wgm@gmail.com>
1. use url crc32 for meta filename.
2. fix not suppert concurrent download, dont save meta.

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

Signed-off-by: Jack <jack.wgm@gmail.com>
improve rangefield, rename m_need_clean to m_need_merge.

Signed-off-by: Jack <jack.wgm@gmail.com>
add comment for async_http_stream.cpp

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

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

Signed-off-by: Jack <jack.wgm@gmail.com>
Update README.md
Merge pull request #11 from avplayer/master

一些更新