Init command

I've added an init command which makes it easy to convert an existing gem to use Octopress Ink. Here's how this works:

```sh
$ cd some_plugin_gem
$ octopress ink init .

      create  ./assets/images/
      create  ./assets/fonts/
      create  ./assets/pages/
      create  ./assets/files/
      create  ./assets/layouts/
      create  ./assets/includes/
      create  ./assets/stylesheets/
      create  ./assets/javascripts/
      create  ./demo/_layouts/
      create  ./demo/_posts/
      create  ./demo/index.html
      create  ./demo/Gemfile

To finish setting up your Octopress Ink plugin:

1. Add gem requirements to your gemspec:

spec.add_development_dependency "octopress"
spec.add_runtime_dependency "octopress-ink", "~> 1.0", ">= 1.0.0.alpha.44"

2. Add a plugin template to your gem, making changes as necessary:

require "octopress-ink"

Octopress::Ink.add_plugin({
  name:          "Food Fight",
  slug:          "food_fight",
  assets_path:   File.expand_path(File.join(File.dirname(__FILE__), "../assets")),
  type:          "",
  version:       FoodFight::VERSION,
  description:   "",
  website:       ""
})
```