Xattr 20140827

Added methods to IoCTX for setting, getting and removing extended attributes of objects.
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I am very sorry that this is competing with the feature branch "CzBiX:xattr" by now.  However, I had been working on it for quite a time but I didn't manage to push my work to my repository until now because I'm still learning git and GitHub.
Nonetheless I propose my additions now as I don't want to just throw them away.  Please consider them.
Thank you very much.