Removed need to require Octopress gem

The Octopress gem needs to require Octopress ink for the `docs` command to work. Octopress Ink can function independently of the Octopress CLI and ideally it should.

The only feature it depended on from the CLI was the ability to read the `_octopress.yml` config file. I've made it possible for this gem to read the config files from the Octopress gem if present, otherwise, it will read `_octopress.yml` itself.

Why? Because If Octopress require Octopress Ink, and Octopress Ink requires Octopress, well; that causes Bundler to be displeased. So as far as I can tell this is the only way for Octopress to require Octopress ink for its docs. I'd rather avoid that dependency, but not enough to kill the niceness of build in custom docs.