Octopress 3.0 Plugin & Theme Scaffold Logic

Usage:

```bash
$ bin/octopress scaffold plugin my-cool-plugin-name
$ bin/octopress scaffold theme my-rip-off-of-the-classic-theme-with-a-different-color-lol
```

Tasks:

- [x] Setup commander-based command in `bin/octopress`.
- [x] Setup command structure in `lib/octopress`
- [x] Write logic to scaffold a new plugin's directories
- [x] Write logic to scaffold a new plugin's Rakefile
- [x] Write logic to scaffold a new plugin's Gemfile
- [x] Write logic to scaffold a new plugin's integration with Octopress's Rakefile
- [ ] Write logic to scaffold a new plugin's integration with Octopress's plugin installation
- [ ] Write specs

Gist for reference: https://gist.github.com/parkr/5745276