git hooks for IPython

add these to your .git/hooks

For now, we just have post-checkout and post-merge, both of which just update submodules, so make sure that you have a fully synced repo whenever you checkout or pull.

To use these hooks, you can symlink or copy them to your .git/hooks directory.

ln -s ../../git-hooks/post-checkout .git/hooks/post-checkout
ln -s ../../git-hooks/post-merge .git/hooks/post-merge