Add git-j, a script for jumping around git branches.

Using git-j, you can easily jump to branches, jump back, or walk up
your parent branches.

git-j can be used in the following ways:

git j <branchname>  - Jumps to a branch
git j -             - Jumps to the last checked out branch
git j ..            - Jumps to the parent branch (stopping at master)
git j history       - Show the stored jump history
git j <number>      - Jump to the numbered item in the history

On first usage, this will install a post-checkout hook to the repository.
It will not overwrite an existing hook, but it must be present for git-j
to do its job correctly.