support mp5hd stream code.

[zhangn1985]

ykdl http://www.acfun.tv/v/ac3253653 -F TD
Traceback (most recent call last):
File "/home/zhangn1985/source/ykdl/bin/ykdl.py", line 148, in
main()
File "/home/zhangn1985/source/ykdl/bin/ykdl.py", line 127, in main
info = parser(u)
File "/home/zhangn1985/source/ykdl/ykdl/embedextractor.py", line 50, in parser
return s.parser(vid)
File "/home/zhangn1985/source/ykdl/ykdl/extractor.py", line 19, in parser
info = self.prepare()
File "/home/zhangn1985/source/ykdl/ykdl/extractors/youkubase.py", line 35, in prepare
stream_id = stream_code_to_id[stream['stream_type']]
KeyError: 'mp5hd'