fix zsh completion

This `_hub` completion script should correctly wrap modern versions of
the `_git` scripts distributed by both zsh (native) and git
(bash-based).  The bash-based version could use a bit more love to
complete arguments to the actual subcommands.  Right now it can only
complete the names of the subcommands themselves.

The `compdef` that was previously emitted by `hub alias -s` prevents the
new `_git` script from working, so it has been eliminated.

Fixes #231
add hub commands to `hub help --all`

While exploring the bash-based completion script for zsh, I initially
implemented this so that the bash script would see the new commands.
Obviously this didn't work because the completion script invokes the
real `git` command, not `hub`.  But I think the feature is nice for
users.