Alter default location of git-options

By searching for `git-options` in the same directory as `pcmpl-git.el` by default, it will be possible for me to create a [MELPA](https://github.com/milkypostman/melpa) recipe that works out of the box for end users. All of a package's files are installed to the same directory, and defaulting to searching for `git-options` in `data-directory` means that completion of subcommand options will not work without the user customizing the `pcmpl-git-options-file` variable.

Thanks for this library, I use it pretty much every single day, and it helps a lot when I can't remember git's consistently obscure option names. =)