support for overlayfs

I've added support for overlayfs (in place of aufs).  

The default is always aufs, so I think nothing should break as a result. Overlayfs support can be enabled by passing a "-f overlayfs" argument to cvmfs_server (i.e. "cvmfs_server mkfs -f overlayfs"). 

Behind the scenes, this adds a "CVMFS_UNION_FS_TYPE=overlayfs" line to server.conf under repositories.d which causes "cvmfs_swissknife sync" to rely on overlayfs code instead of aufs code (both are implemented in sync_union.cc). I think this way it should be possible to have both overlayfs and aufs based repositories on the same machine (not sure why one would want to, but I didn't see any reason not to support it). 

I welcome any questions or review before merging...

Josh.