Add Capabilities bit support to docker layers

This adds support for storing capability bits (via xattrs) in docker layers.

I tested by running the mattdm/fedora:f19 image, installed httpd which has an attr in /sbin/suexec, commited the container to an image, started a container based on that image and verified (with the attr command) that /sbin/suexec still was set.

Note, this replaces https://github.com/dotcloud/docker/pull/2048 which seems to have a lot of unecessary commits in it.