Topic/atomic fetch

Replace the current <op>_and_fetch type of atomic operations with fetch_and_<op> (in other words returning the old value instead of the new one).

This patch is based on the [devel list discussion](http://www.open-mpi.org/community/lists/devel/2014/07/15327.php). Unfortunately, this patch is not complete. This PR is just a placeholder where volunteers can push their changes. There is not much left over: all __asm__ operations are converted, but someone needs to take a look at the generated asm code.

@goodell and @hjelmn please review.