Bug 764732 (GLUSTER-3000)

OK, I admit it, I did this partly to get bug 764732.  Seriously, though, since extensibility is one of the most significant benefits of GlusterFS relative to most alternatives, I think it's imperative to facilitate actual extension without needing to have a full GlusterFS source tree available (which violates many projects' packaging guidelines).  For CloudFS I've managed to build translators "out of tree" using simple makefiles and only headers installed as part of the Fedora glusterfs-devel package.  Ideally this same work would also apply to Gluster's own packaging, to Debian, etc.  Also, the particular files that are part of the "external API" represented by the -devel package - especially those necessary to build any translator - should be carefully chosen and clearly identified (e.g. by placing them in a single directory).  Right now that's not the case, with some essential header files buried e.g. in contrib/uuid and every include directory mingling essential with non-essential files.