Add script for updating the external libraries.
Squashed 'external/luabind/' changes from 9157d14..c891641

c891641 Make bind_class_info idempotent (can call repeatedly without breaking)

git-subtree-dir: external/luabind
git-subtree-split: c8916413b747eabf09615aca658445cec389e4d4
Merge commit '2550253aaae18d269e351e290c34dc89367821f6'