update the copyright header
make rpmkit.utils.longest_common_subsequence returns str if first argument is a str
minor doc text updates
add an argument to specify root rpms to rpmkit.rpmutils.walk_dependency_graph
add a function to list required RPMs recursively not required by others like saferecursiveerase() in depgraph: https://github.com/sugaken-r3/depgraph/blob/master/depgraph.py