Delete leftover krbdev scripts

Remove krb5-rt-id, post-receive-rt, and ssh-as-krbsnap, as they are
unused in the Athena repository.
Add a prepare-commit-msg hook

Emulate the old svn behavior by adding a hook to pre-populate
the commit message with any additions to the changelog file
Do not run hook if commit mesage is specified

If the commit message is specified on the command line,
don't run the hook
Allow run() to not fail; add git config helper

Modify run() so the caller can decide that nonzero exit status
need not terminate the hook.  Add get_git_config() helper to allow
for easy access to a config key with a default if necessary
Allow repositories to opt-out of whitespace check

Modify update() hook to bypass whitespace check if repository
is configured to do so
Merge branch 'master' of https://github.com/mit-athena/repository-configuration
Delete krbdev versions of hooks

Remove the krbdev post-commit and commit-msg hooks, which are unused.
Add README file for client hooks

Add a README file explaining what this directory is for.