git hooks for IPython

add these to your .git/hooks

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

To use these hooks, run ./install-hooks.sh. If you havn't initialised and updated the submodules manually, you will then need to run git checkout master to activate the hooks (even if you already have master checked out).