Added '--xattrs' option which writes metadata to the file's extended attributes using a youtube-dl postprocessor.
Works on Linux, OSX, and Windows.
Merge remote-tracking branch 'epitron/metadata-pp'

Conflicts:
	youtube_dl/PostProcessor.py
[xattr] Coding style
Move postprocessor into its own package
[orf] Remove unused variable name
Split postprocessor package into multiple modules
[xattr] Write bytestrings, not characters
[xattr] Always output a warning message on errors
Move check_executable into a helper ufnction
[xattr] Rework

In particular, explicitly require NT before trying ADS, and do not try to parse process output that may be localized.
[xattr] Always use UTF-8

On Windows and other systems, other encodings would break when trying to encode non-ASCII characters.
Simply use UTF-8, like every sane system.
[xattr] Correct on Windows