move file.hpp & storage_interface.hpp to avhttp filter

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revert avhttp.cpp

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Merge branch 'master' of github.com:avplayer/avhttp
disable warning 4250 on msvc

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add missing boost.format header

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add bitfield.hpp

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update vc2012 project file

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change range_to_bitfield support to bitfield

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update avhtt.hpp header

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add comment to function description

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add example multi_download.cpp

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add a open overload

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fixed calc range on timeout

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add file_size

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add file_name

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improve example multi_download.cpp

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update unit to 1024

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fixed done state

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improve multi_download

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checke file open

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