Support --storage-opt for aufs graph driver

This patch alllows user to pass extra mount options to aufs graph driver via the daemon's `--storage-opt` flag. 

Our use case is that we want to test `dirperm1` mount option ( http://aufs.sourceforge.net/aufs3/man.html ) which only available to kernel with newer aufs patches to see if it can mitigate the permission bug with aufs (https://github.com/docker/docker/issues/783)
can someone explain how we get `50` here ?